wait a sec,which model are you talking about.(ive seen three types all have the ir sensor)
the part im talking about is the ir sensor( infra red ) if this is gone no disc shows in the system.(the beam is broken when the disc is inserted)
does it load ok when assembled.(it will still load a disc even if the drive isent paired to the system,but it wont load a disc if the sensor is gone)
if the reciever is gone then the disk still shows in the system after pressing eject.im sorry if this is obvios.
i dont use the method you mention, as this has its own problems on assembly of the unit.
as for the ribbon cable they can get broken,if the latch is gone you will deffinitly have problems there.the latch presses the cable againts the contacts it also centres the cable.(but the motors in the drive will still work as its powers taken from somewhere else)
also if this cable isent inserted properly when powered on it can destroy the controller board.
do you still have the latch part?
it also sounds like your ps3 is in a bad way,and been manhandled.i know they can take a bit of abuse but still the electronics in these is fairly fragile.and im sorry your having such bad luck with this one.
EDIT:have you tried the recovery menu by any chance,i know you have probably tried all the obvios stuff already,but i like you have came to the possibility that the drive isent paired to the system,have you asked the guy if he ever had a disc show in the system?