I have recently tried flashing LineageOS on a Nexus 5.
As @MarkG_108 said I still happen to have the UT bootloader I think (as a picture comes up on booting that Nexus 5 is powered by Ubports or something like that).
I ran in to multiple issues reinstalling stock Android (to test that first) but finally it worked fine, I downloaded the .img files from here and flashed each .img file individually with fastboot (like "fastboot flash bootloader bootloader.img" or "fastboot flash system system.img" etc. I think from the folder that contained the images). I think I got the idea from this guide. This booted to normal Android I think with no issues (?). I had some issues connecting to fastboot and adb and I don't remember what I did to fix it but I just kept looking up what to do (like searching "how to fix fastboot devices not showing anything").
So, I don't know how different Nexus 7 is from Nexus 5, and am not sure if you tried to flash all the .img files (or if that is needed) since @Elowan said "installer complains about missing some images".
I'm thinking you should be able to do something to get things working though so I hope you can keep trying and talking with us and something works out!