Given that you stated the screen has artifacts when fully opened leads me to believe that the ribbon cable that from the bottom of the 3DS and the top screen has been damaged. It's routed through the hinge on the 3DS, which would explain why opening and closing the device would change things. Unfortunately, to fix this issue, you'd need to replace the entire top screen. This guide would show you how to replace the top screen. Speaking on the issue of the console not turning on, I can't give you a definitive answer. I've seen units that won't turn on if the device has enough screen damage, but it's a shot in the dark whether or not replacing the screen will fix it.

Sounds like the BIOS is now corrupt. By removing power from the computer, the BIOS is only half written. Without a proper BIOS, your computer will be unable to boot in any fashion. Unless the BIOS chip is socketed or you're willing to solder a new one, your only option is to replace the motherboard. If it says not to turn off, don't turn it off.

If done by Google or one of their subcontractors, absolutely, your phone still carries the same water resistance. If you did it, it's debatable. All of that water resistance hinges on the quality of the seal around the screen. If you used the same high quality adhesive and made a complete seal around the phone's screen that would do the trick, but anything less and I wouldn't trust it.

I made a guide for the J3 2018’s screen the other night, I repaired the T-Mobile model. Here's the link! Samsung Galaxy J3 (2018) Screen Replacement