How did it “suddenly die”? Do you get anything at all? Have you reset the PRAM? I would also double check all the connections, specifically the LCD cable and make sure it didn’t pull loose from the BACK OF THE LCD. I would measure the voltage to the connectors as well, just to make sure the connectors are actually getting power. 2010 iMacs are notorious for GPUs failing as well, just an FYI
Well without looking it up, phones generally open one of two ways; The back comes off somehow (Screws, snaps, or adhesive) and you can get at the internals through there, or you have to pull the LCD off which would be generally held down with adhesive, that you would need to heat and separate.
There’s a resistor (r1506 on a1475)you can remove from the LTE ipad that will disable LTE and make it Wifi only but it’ll bypass the icloud lock. That would be your best course of action imo
If all the cables are connected correctly, I’d try a new front camera flex just to be sure. 7s boot loop for pretty much any little issue, just try a couple things if you have parts around
Have you tried a new battery or powered it with a PSU just to see if it will turn on? Regardless, before you do anything else I’d inform them of what you found.