Screen unresponsive after replacement and flickering

It might not be the screen, it could be the battery. My old phone got hot and the display would sometimes short out but I got a new battery and it still works like a charm even though it's a iPhone 4

But also it could be a defective screen and that it's not on property.

If the screen is fine while not seated, then most likely there is something appling pressure to it from the board side.

Try assembling the phone without the cover plate of the lcd connectors and check all the other screws and plates if they are in place.

ALSO check the rear antenna cable, it is protected by a metal plate and that plate can be easly missplaced if any speaker repairs were attempted. the plate must go underneath the speaker or it will apply pressure on the LCD.

I have had this same issue twice with a iPhone 6 plus. I'm on hold currently with Apple booking a time to replace my current phone for the 4th time. I'm not having a good run with my 6 plus. No drops no bending, no water. Just a plain defective phone's, thank god for Apple Care.