I have a Pixel 3a as well (16.04) and I don't have an issue with the flashlight- it works fine, so I wonder if it's an issue with the light itself. Were you able to test everything before you installed UT? I'm no expert (actually just started using UT myself) so this isn't advice but I wonder if you would be comfortable with flashing Android back on it to see if the light works then flashing back to Ubuntu Touch to see if it's a hardware issue. Maybe there's an easier way to check it though that someone else knows.

As for MMS, I've had an issue with that as well. I've been able to get texts working fine, but not pictures (whether from iPhone or Android) but like I said, I'm a beginner and sending pictures through text isn't that important to me anyway so I haven't tried a whole lot there.

@lakotaubp Nice, that's actually not that bad of a workaround. I couldn't resist going back to UT to give it another chance so I flashed it back onto my device and texting/calls worked fine on 2g/3g for a day and a half, though still no mms. Then yesterday I stopped getting texts again. Luckily, I have a work phone too so I can just periodically switch airplane mode on/off and send a text to verify. Thanks!

Same issue here with Mint Mobile (T Mobile). At first, unable to receive calls at all but calling out and texting worked intermittenly. After a "factory" reset to set API back to default, (I did mess around with API settings to try to get calls to work) the past couple days I could send/receive calls, and send texts, all very quickly and easily but couldn't receive texts at all. Switching to 2G/3G didn't help. After flashing android back onto my Pixel 3a, all of the texts I didn't receive on UT flooded into my text message inbox. Bummer since I LOVE the UI for Ubuntu touch, but need more consistent reliability.
I will definitely be keeping updated though. Once a solid solution is found, I will be back on Ubuntu Touch without a moment's hesitation.