Well, with the little amount of information, I guess I'll just give you my personal opinion. Those phones arent built with best materials. This makes the phone cheaper to purchase but also have a low life of usage. I would just buy another one.
you can't replace the home button on the 6 series. they are encrypted per device. once you break that then its through. the phone will error out and bootloop. if you used metal tools , instead of plastic, you could have shorted out the motherboard. any other details on how you did the repair that could help us come to a better conclusion
You should be able to switch back by going to Default Applications in setting and switching the dial pad back to its original program. Or back-up and do a factory restore, that should work too
Sounds like you answered your own question. Seems like the motherboard to me, assuming the parts you used were the same parts for that model. How sure are you that you used the correct parts?
Its a nextbook. Very low quality Android tab. They ALWAYS have issues. With the little amount of info you gave me, I'd say call it quits on the nextbook and consider a Nexus or a Kindle as a replacement
Definitely sounds like a bad battery. This can happen a couple ways. 1. Bad cable over/underpowering the battery. 2. Water damage 3. Left unused for a long time. There's tons more I could list. Regardless it sounds like it needs a new battery. However, Iconia tabs are stupid cheap nowadays, I'd consider just buying a new one.