You probably shorted the connectors in there. Please refer to this guide: New Nintendo 3DS XL (2015) 메인보드 교체

Yes, the Nexus 6P's battery has a temperature sensor pre-installed.

A hard reset might be able to fix this. (1) Put new batteries in the camera and remove the memory card. (2) Double click the on/off button and the camera will turn on and then off. (3) Hit the on/off button once more, and zoom in. Hopefully, the camera will start normally. This might be able to fix it. Another solution would be removing the battery and SD card, replacing it after letting it sit, powering on the camera while holding the function and shutter buttons, and giving the lens barrel a couple of face-down light taps on a table.

Well, it really depends on what you replace it with. If you get an OEM backplate with proper adhesive, you're probably fine. If there's a third party backplate and weak adhesive, you've probably lost your waterproofing (submersion) but you're probably still fine for splashes.

Your device probably has a loose connection somewhere. How to repair Samsung J5 (2016) LCD screen

I highly doubt there is a way to add a GPS module to a MacBook. If you are worrying about tracking a stolen device, Find My Mac through iCloud works fine.

First off: do you have an Apple Music membership? It is only an option if you are subscribed to Apple Music. Secondly: are you signed in with your Apple ID on all your devices? You might need to sign in again if you have an Apple Music membership.

Go to Settings > General > Accessibility > Increase Contrast > Reduce Transparency and disable.

This is a pretty common problem with the original 3DS, actually. The problem is either the speaker or the camera, and I'd be willing to bet money that it's the speaker. Nintendo 3DS Speaker Assembly Replacement

The extent of the issues here makes a repair pretty much useless. You'll probably want to get a new phone.