I did own an early K100RS, one of the first ones in Vancouver and certainly the first in Victoria BC. It did have the thrust washer between the carrier and the engine. They tend to stay stuck to the carrier from the oil between the parts, so may be hard to see, but you seem sure that it is missing.
Don't know what to say about the different thicknesses, them thrust washers are usually to take off play/wear between components. Maybe get a deflection gauge and check for play without the washer in place, then do the same with the different washers and see what happens.At this time last year I could have done a few measurements on my bike out of curiosity.
I fail to see how the .25 mm difference would affect much of anything. Could it be to back up the carrier a bit in case of a wear groove from the seal, thus giving a new surface for that seal to run on, or slightly more compression to the rear O-ring.
But I went and checked the K75 parts fiche, washer is shown but not listed, could your bike have been changed around some? There may be a part number on that carrier, if there is maybe go in Max's parts fiche and do some referencing by punching that number into the parts finder, would tell you what it is coming from.
But....tell us about that clutch problem, I have a feeling that it may be all related.