Just to follow up on this post, I played around with more Gcam builds and I think I found a good one with working view finder for night sight. I haven't played around with it much but the few shots I took look great to my eye. The slow motion behaves like fu24's build that I described in the post I quoted. Everything else seems to be working fine.
For anyone that wants to try, download this Gcam build:
https://f.celsoazevedo.com/file/gcamera/GCam_6.1.021_Advanced_V1.5.190418.1850.apk
And download the config file I made and posted here:
https://forum.xda-developers.com/showpost.php?p=79582691&postcount=2316
Instructions on how to install config file:
https://www.celsoazevedo.com/files/android/google-camera/f/settings09/
I basically downloaded the G6 config file, since the one for our phone causes the app to crash, and made some changes to make it work best on the G5 (ie changing the base config/model and enabling the buffer fix etc.). I made it so that 'Long Press' is the way to switch between the normal and wide lenses. You can set it to cycle instead in the settings.
Sorry for getting OT but just thought I'd share. Plus if you want a fully functional camera app, you'll need a different one than stock atm, and this might be the best for our device.
If anyone feels that this should be posted somewhere else, feel free to delete or move it.
edit: Not sure which build I like best between Arnova's advanced, BSG's original v2e or fu24's build. Arnova and BSG's builds seem to have similar white balance, but Arnova's exposure seems off sometimes... seems to overexpose. fu24's white balance seems a bit different than the other two, sometimes it looks better, sometimes it looks ever so slightly green. Arnova's build is the clear winner for night sight... not only does the viewfinder work, but the images look far better. It seems to be the only build that properly processes night shots and the results are impressive.