Stopped fermentation was caused by fluctuating temperature and too quick cooling after pitching the yeasts.
We salvaged it to sellable beer but it wasn't great.
Give it a taste test and proceed as normal brew, in a best case scenario you will have sweeter beer.
Restarting a fermentation is tricky and unreliable so I wouldn't do it. You may have issues with carbonation so it may end up flat or, if you can, force carbonate it.
Try to find what caused it, so you don't have to deal with it in the future. I can't tell what it is from your post - yeasts, temperature, bad measurements...
I don't see anything obvious so that's it, I don't have experience with this hydrometer so I can't help with that (bad calibration or some issue there).
Tldr: not much to do now, try to find out what caused it for future brews.