Early 91's have the Bosch unit (has external cooling fins), later cars have the Mitsubishi (no visible fins), both used the same belt. Not exactly interchangeable, plug connectors into units are different.
Original alternators in good shape will not give you problems in shredding belts.
It doesn't matter what alternator came with the car, just what is in it now. Many aftermarket alternators use a different pulley that is not quite right, making keeping a belt on a challenge.
This doesn't sound like the usual belt issue the Capri's have. Constant squealing then breaking (from overheating) then wearing on one side seems to indicate an alignment issue. If the water pump was starting to fail, that pulley could be out of alignment with the crank and alternator pulley. If the alternator mount has worn, that pulley could be misaligned. If either water pump or alternator is binding internally, that could also wear the belt quickly.
Pull the belt, use a straight edge to verify pulleys are in line. While the alternator is loose, check for play at the mounting joint, should be 0 play