It sounds like the connector for the digitiser is either not quite making contact properly, I would just try removing it, giving both sides a quick clean with isopropyl alcohol and reseating it again, hope you get it sorted.
The only times I have had this issue either: - The connector wasn't fully engaged and just needed popping off and re-seating - Small tear in the cable for the connector - Bad part there is a slim chance that the connector on the logic board itself was damaged in the original incident but I think I've only seen 1 phone with this issue and I put it down to a DIY fix gone wrong.
Hi, It is possible, what you would need to do, the guide is here: iPhone 4 (CDMA/Verizon) Screen Replacement step 13 is removing the cover where the connectors are and the connectors are identified in step 15 the only issue I see is that if you don't remove the display assembly the 2 cables you will be unplugging and replacing may get in the way as they lift up and point out of the top of the phone, you can then plug in the 2 new connectors but will need to support the display assembly you are testing, turn on the phone, turn it back off etc. good luck :)
as apple like to keep things under their own banner as much as possible i wouldn't be surprised if aftermarket spares aren't 'official parts' I've had a few from wefixmac and they look/feel the same and have had no complaints so far. I imagine there will be some dodgy ones out there, try and buy from the most reputable place you can find/afford and you should be fine.
It would be worth in the first instance checking the cable from the LCD to the main board is attached properly at both ends, if it is you will need a new LCD [as long as the digitiser itself isn't cracked] The LCD/screen itself is the same for the wi-fi and 3g versions, it is just the digitiser that is different between the two with the 3g version having an additional antenna for the 3g. hope you get it sorted
It shouldn't be digitiser related, video chip is a possibility I suppose but not sure how you could test for this. I've found you can leave at least some of the rubber dots/spacers out if they're missing, real danger is if they move it can lead to them pushing on the screen when reassembled and this will usually distort the colours where it is being pushed. Did you check both ends of the video cable as I have seen some come loose despite the tape still being seated over the top of them, sounds like the LCD as you said.