How to fix? Touchsreen fail after drop ,(Motherboard related)

If it is an issue with the motherboard, then you would have to be able to solder on the board which is very difficult. If you tried other screens then it is most likely something with the board. Check the digitizer and LCD prong connectors on the board and see if they are bent, if they are bent, then that is your issue and you would have to be able to solder a new connector onto the board, which again, is very difficult. You can purchase the connector from mengtor.com, which is were i purchase most of my parts for my repair shop. If you can get your hands on an OEM screen if you don't already have one , take some isopropyl alcohol and wipe the inside off the phone VERY lightly, i would recommend 99% purity so that you don't damage anything. All you are trying to do is remove your fingerprints. Do not wipe the battery because you will smear the letters and make it obvious that you were in the phone. Just wipe your fingerprints, they are usually on the metal only. So like the metal panels on the screen and the cover for the flex cables. Once you do that, take it to apple and see if it can be repaired. Another thing you can do is get your hands on an OEM screen and toss that on there and make sure everything is cleaned off any finger prints to erase all evidence, then put in on, take it to apple and say it doens't work.