S2 had DEALER fitted keyless. S3 had proper OEM keyless.
#1, they are all that way. its teh adjustment in them that makes them do that.
#2, that is all to do with mounting.
#3, that CAN be done. you need to find the drive wires for the STOCK control unit then you will have to rework your alarm to control it. if you are unsure what I just said DONT attempt it.
not possible. you would need a custom solution for that one.