I had the same problem. Check both ribbon cables for the screen. They should be the exact same length after installing it through the metal housing. When I had the problem, the cause was a torn ribbon cable. Check there first.
yeah you would have to purchase another display assembly. unless you're a very professional technician that can fix tiny ribbon cables. i had the same problem as you did, and had to purchase another assembly...
From what I heard, the screws are completely needed. It could so happen to be that one of the screws connect the steel frame (GPS and WIFI frame) to the motherboard so you can get a signal. You can google up where the screws go.
Is there a place where I could purchase that black piece? When I disassembled my phone, I saw some black piece fly out and I thought it was something else, so I didn't worry about it. Now I can't find the piece...
You can put it all together, but if you don't attach them with glue, dust could possibly get in between the digitizer and LCD. In my opinion, companies are selling them separately to rip people off. It happened to me.