Ok, Excellent news! I successfully flashed Ubuntu Touch (without any errors) on the Nord N100 Metro MODEL BE2015!

I'm gonna give detailed instructions here on how I did it. You can ask AI or do a Google search to get the finer details of each instruction if needed.

Step (1)
Download the ADB Platform Tools for Windows (This allows Android control from Windows terminal via usb cable), as well as the Qualcomm_HS-USB_QDLoader_9008_Driver for windows (This allows Windows to recognize your Nord N100 when plugged in), install them both.

Step (2)
Download the bengalm_14_O.04_201221 mod (This zip contains the MSM Download tool for flashing the .ops Oxygen OS 10.5.5 rom also included, which has been specially patched to work with the Metro MODEL BE2015)

Step (3)
Activate Developer Options from your phone by going to Settings -> About Phone, and tap on Build number 7 times.
This makes the Developer Options show up in your system settings.
Find USB Debugging and activate it. Also find and activate OEM Unlock.

Step (4)
Start up the MSM Download tool; When Windows Defender try's to stop you, hit 'more info' and continue anyway, choosing user type 'other'.
Now that it's waiting for device, go to the directory where you unzipped the Platform Tools, right click in empty space next to the files and select open in Terminal (In older versions of windows I believe it is Powershell Terminal). Use commands:

./adb devices
Shows connected device serial number. (make sure you allow the computer from your phone screen to connect)

./reboot bootloader
reboots your Nord N100 into fastboot mode so the MSM Download tool connects. Make sure your Qualcomm drivers are installed for this to work.
As the device shows up in MSM Download tool, hit the Start button, and the rom starts flashing the device.... til it's finished and now OOS 10.5.5 should be on your device.

Step (5)
As your new OOS boots up, go through the new device startup, but don't connect it to internet or it'll try to update itself to a newer version, which we DON'T want.
Go to Settings -> About Phone -> tap Build number 7 times to activate Developer Options.
Go to Developer Options -> activate USB Debugging
Also in Developer Options -> Disable Automatic Updates
Now you can connect to Wifi, then go to Settings -> Accounts
Add a Google account to the device.
Now back to Developer Options -> enable OEM Unlocking (This is greyed out if you don't add the Google account)

Step (6)
Go to the Platform Tools terminal again with device connected via USB
./reboot bootloader
puts it in fastboot mode
./fastboot oem unlock
brings up a menu on the phone. use volume down key to select unlock bootloader and press the power button to confirm.
Now your user data is erased, and we have to run a loop again, but the bootloader is now unlocked.

Step (7)
Phone restart, and going through the new device startup.
Don't connect to the internet yet, or it'll try to update.
Go to Settings -> About Phone -> tap Build number 7 times to activate Developer Options.
Go to Developer Options -> activate USB Debugging
Also in Developer Options -> Disable Automatic Updates
Now you can connect to Wifi, then go to Settings -> Accounts
Add a Google account to the device.
Now back to Developer Options -> enable OEM Unlocking (This is greyed out ALREADY, says OEM is already unlocked), So connecting internet and adding a Google account may be unnecessary here, but that's what I did.

Step (8)
Download UBports Installer (0.11.2), I actually did this on my Linux Mint OS, and downloaded the ADB Platform Tools for Linux as well. Not sure if this will make a difference, Tho I'm much more comfortable on Linux. You can try it on Windows tho and may be fine.
Connect your device via USB and use Platform Tools command
./adb devices
boots into fastboot bootloader, which should now show that the bootloader is unlocked.
Run UBports Installer. It should auto-detect your device, saying Billie2 even tho it's Bengal now. Select to install Ubuntu Touch.
It asks you to make sure you have either the OOS 10.5.2 or 10.5.3, but we've now got the 10.5.5 which worked for me, and so I ignored that, and went ahead affirming My device is unlocked.
....next....
I chose Channel 24.04/stable
check all three boxes "Wipe Userdata", "Partitioning", "Bootstrap".
Then hit "OK"
and....
Bing Bam Boom!!! It starts the process of installing showing the status bar and the fancy UBports robot. Just sit back and relax. When it finishes, your phone should automatically boot into your fresh newly installed Ubuntu Touch!!

Step (9)
Test out the new operating system.
I found that I couldn't connect to wifi. Thought it mighta been a glitch, but after playing around then powering it off and rebooting, I was able to connect the wifi. So far so good.

Good Luck my Ubuntu Mobile Linux friends!

@rocket2nfinity
After testing over the past couple days, it seems the sound doesn't work for whatever reason. Also, just today I rebooted the device and now it won't connect to wifi. I enter wifi settings and has the options of "Connect to hidden network" or "Previous networks", but no hotspots come up. I remember this happened when I first installed, but then I rebooted and was able to connect wifi, so I'm not sure why it isn't working now. I rebooted again and same thing. Tried to put in the hotspot details via "Connect to hidden network" but that's not working either.

When the wifi was working however, I was able to get Waydroid installed, and installed F-droid app store and some other android apps. All the apps seemed to work, aside from the sound not working. Camera worked tho.

I installed this Ubuntu Touch to be a privacy device, and thus have no intention of ever putting a sim card in it. So I haven't and won't be testing that.

Ok, Excellent news! I successfully flashed Ubuntu Touch (without any errors) on the Nord N100 Metro MODEL BE2015!

I'm gonna give detailed instructions here on how I did it. You can ask AI or do a Google search to get the finer details of each instruction if needed.

Step (1)
Download the ADB Platform Tools for Windows (This allows Android control from Windows terminal via usb cable), as well as the Qualcomm_HS-USB_QDLoader_9008_Driver for windows (This allows Windows to recognize your Nord N100 when plugged in), install them both.

Step (2)
Download the bengalm_14_O.04_201221 mod (This zip contains the MSM Download tool for flashing the .ops Oxygen OS 10.5.5 rom also included, which has been specially patched to work with the Metro MODEL BE2015)

Step (3)
Activate Developer Options from your phone by going to Settings -> About Phone, and tap on Build number 7 times.
This makes the Developer Options show up in your system settings.
Find USB Debugging and activate it. Also find and activate OEM Unlock.

Step (4)
Start up the MSM Download tool; When Windows Defender try's to stop you, hit 'more info' and continue anyway, choosing user type 'other'.
Now that it's waiting for device, go to the directory where you unzipped the Platform Tools, right click in empty space next to the files and select open in Terminal (In older versions of windows I believe it is Powershell Terminal). Use commands:

./adb devices
Shows connected device serial number. (make sure you allow the computer from your phone screen to connect)

./reboot bootloader
reboots your Nord N100 into fastboot mode so the MSM Download tool connects. Make sure your Qualcomm drivers are installed for this to work.
As the device shows up in MSM Download tool, hit the Start button, and the rom starts flashing the device.... til it's finished and now OOS 10.5.5 should be on your device.

Step (5)
As your new OOS boots up, go through the new device startup, but don't connect it to internet or it'll try to update itself to a newer version, which we DON'T want.
Go to Settings -> About Phone -> tap Build number 7 times to activate Developer Options.
Go to Developer Options -> activate USB Debugging
Also in Developer Options -> Disable Automatic Updates
Now you can connect to Wifi, then go to Settings -> Accounts
Add a Google account to the device.
Now back to Developer Options -> enable OEM Unlocking (This is greyed out if you don't add the Google account)

Step (6)
Go to the Platform Tools terminal again with device connected via USB
./reboot bootloader
puts it in fastboot mode
./fastboot oem unlock
brings up a menu on the phone. use volume down key to select unlock bootloader and press the power button to confirm.
Now your user data is erased, and we have to run a loop again, but the bootloader is now unlocked.

Step (7)
Phone restart, and going through the new device startup.
Don't connect to the internet yet, or it'll try to update.
Go to Settings -> About Phone -> tap Build number 7 times to activate Developer Options.
Go to Developer Options -> activate USB Debugging
Also in Developer Options -> Disable Automatic Updates
Now you can connect to Wifi, then go to Settings -> Accounts
Add a Google account to the device.
Now back to Developer Options -> enable OEM Unlocking (This is greyed out ALREADY, says OEM is already unlocked), So connecting internet and adding a Google account may be unnecessary here, but that's what I did.

Step (8)
Download UBports Installer (0.11.2), I actually did this on my Linux Mint OS, and downloaded the ADB Platform Tools for Linux as well. Not sure if this will make a difference, Tho I'm much more comfortable on Linux. You can try it on Windows tho and may be fine.
Connect your device via USB and use Platform Tools command
./adb devices
boots into fastboot bootloader, which should now show that the bootloader is unlocked.
Run UBports Installer. It should auto-detect your device, saying Billie2 even tho it's Bengal now. Select to install Ubuntu Touch.
It asks you to make sure you have either the OOS 10.5.2 or 10.5.3, but we've now got the 10.5.5 which worked for me, and so I ignored that, and went ahead affirming My device is unlocked.
....next....
I chose Channel 24.04/stable
check all three boxes "Wipe Userdata", "Partitioning", "Bootstrap".
Then hit "OK"
and....
Bing Bam Boom!!! It starts the process of installing showing the status bar and the fancy UBports robot. Just sit back and relax. When it finishes, your phone should automatically boot into your fresh newly installed Ubuntu Touch!!

Step (9)
Test out the new operating system.
I found that I couldn't connect to wifi. Thought it mighta been a glitch, but after playing around then powering it off and rebooting, I was able to connect the wifi. So far so good.

Good Luck my Ubuntu Mobile Linux friends!

Ok, so I read around the net and figured it out.
The OEM unlock is greyed out until you add a Google account on the phone. I was concerned about having read from someone else that the auto-updates implemented automatically, but once you activate the developer settings, you can find turn of "Automatic System Updates" then safely connect it to wifi, add a google account and then the OEM unlock becomes clickable. Yay!!!

Then I went on to the UBports installer, unchecked the partitioning box, selecting Channel 20.04/stable.

The files downloaded, moved to my device, Some errors showed up on the phone screen when it finished, which I expected, UBports installer told me it was a Success. Then I selected Reboot on the phones options and it seems to have just frozen. I guess it's bricked, cuz it won't even turn off with holding down the power key.

Anyhow, I'll have to see if I can unbrick it, and try again.

@MrT10001
I enabled Developer Options, however the OEM unlocking option is greyed out.

Does this mean I have to go through the ordeal of getting the unlock key from OnePlus?!

@MrT10001
Ok, I'd previously tried it from Windows 11 and it didn't work for me, but I just tried it from Linux after installing all the ADB Platform tools, and it seems to register.

First,
adb reboot bootloader
Which shows it's in fastboot mode on the phone.

Then,
fastboot oem unlock
but it gives me:
FAILED (remote: 'Flashing Unlock is not allowed')
fastboot: error: Command failed

So now what do I try?

@rocket2nfinity

I followed the instructions on my metro model BE2015, and it seems to have installed. Now it shows I am on OOS 10.5.5 but how do I now unlock the bootloader?

I follow the instructions from UB Port installer and after trying
"fastboot oem unlock" is just says "< waiting for any device >" and nothing else happens.
The device was connected ok, as it showed the serial number when using "adb devices" command.

Is there something I'm missing?