You've said you've done the Fuel Distributor, but have you done the Warm Up Regulator? The WUR ultimately controls the fuelling based on how hot the car is, if it has failed, it'll almost certainly cause you massive problems.
You can also try unplugging the 5th injector, these have been known to cause more trouble than they're worth, I run with mine unplugged on my quattro and it starts much better.
Pretty sure there aren't any seats, think they're just cut into the inlet manifold. When I've rebuilt them in the past, I've just done the o rings and I believe top and bottom are the same, thought it's been a few years.
It looks like you have most of the pieces worked out, but you'll also need a AC specific shallow glovebox and lower dash trim as the ac evaporator housing is much larger than the stock mk2 one.
The reason your mechanic has said the bearing has gone is because it's the root cause: The seal in the box has failed because the bearing is allowing the shaft to move up and down, which has unevenly worn the seal.
As for gearbox refurbishments, I can't recommend Gas N Gears highly enough, the guy has been in the VW world for 20+ years and has done around four boxes for me, all faultless. Not cheap, but great workmanship.
