The best thing you can do, is attempt to reattach the two wires from the headphone with the splitter . This probably won't work very well, but it may be worth trying, if they are a good pair of headphones

Usually, it is difficult to upgrade the PSU. You will have to make sure to find one with the right connectors. You could most definitely remove the motherboard, if you can take it apart. If you find a bigger case, PSU upgrade will be significantly easier.

Sounds like it could be a blown fuse. You should rest them will a multimeter, and if you don't get a reading from any, replace them

That would be probably a cellular antenna, but if you didn't see a Sim slot it is a Bluetooth antenna. If you think you damaged it, the I would suggest putting it together, and turning it on to see what it was. If it was something you used, then consider purchasing a replacement (if available).

It looks like it could be a bad screen/scanner. You may have to buy a new one. It is very unlikely but possible it is a software issue that requires a reset. I would back it up the reset it, an if that doesn't work, then I would install a different screen and scanner

Probably, either low quality, or not setup properly. You will probably have to buy a new one. I would suggest attempting to return it, or have it exchanged for a better one, or try to find a genuine one.

That could be a few different things. This makes me think logic board (I have a Android tablet that does the same thing to a lesser extent), but it could also be a worn LCD display (or other technical issue). If it is the logic board, it may not be worth it, but the LCD display may be worth it. You may be able to use it for a little bit, but I would suggest taking it in, as you could entirely remove the issue if it isn't too expensive. If it is too high though, your better off buying a new one