Have you tried a network settings reset? I would try this before diving super deep into repairs and troubleshooting.

I dont know about this device particularly but on many Samsung devices these buttons are a separate piece or attached to a charge port, I would check to see if the cable that connects them is torn, it may need replacing!

Check the settings and make sure auto rotate is turned off

Did you try to do a hard reset on it? A hard reset is using a button combination to initiate a factory reset on the device. I would Google your tablet and look for the hard reset combination, on Samsung devices it is generally volume up, power and the home button and than release the power button when the Samsung logo appears. If that doesn't work I would bring the tablet to a Samsung tech. They are able to reload software on Samsung devices. I know some Best Buys have them, I am not sure where else they are. Hope this helps!

did you reconnect the power button?

It looks a little used, as far as for an orginal look for the apple logo on the cables, if you bought this its most likley not made by apple

You could try to check with your carrier to see if the device is on another account. Im not sure if they can disclose this info though

I would remove all the screws from the board to see if something is causing a short or flexing it. Can you test the device with a different screen?

It could be dirty or damaged disc reader. I would check with Xbox's support site for information regarding cleaning the disc reader or repairing it.

Changing the screen on a Iphone 5C is not too difficult. Guides and parts are available on this website to complete the repair. You could always check out the guide, and if it looks too difficult you can bring it to a repair shop. Apple will repair the screen as well, I would shop around and find the best price and situation for you!