You already got most questions answered there are just a few things I’d like to add:
I guess you are asking if FP2 has a better battery than the FP1(U). I’d guess yes, but we’ll have to some months after first shipment if people report e.g. bloating batteries.
If your asking wheter there is a solution for problems with a FP1 the answer is yes: see Battery Guide.
Fairphone will continue to support the FP1 with security updates for some years so even stuck on Android 4.2 it will still be somewhat secure. With the FP2 they will hopefully get Android updates and they will surely provide security updates for many years.
I’m not an expert on current high end smartphone cameras but I don’t know of any phone (yet) that can make really good photos in the dark or make close-ups without an external objective lense.
So to answer the title-question:
If you need a phone and you set value on sustainability, then the FP2 is the best choice.