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
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.