Tested the switch like you suggested and everything matches except for one thing: There is no contact between R1 and A in "medium position. In fact the R1 is left out in all positions. That doesn't seem right.
I don't know why this happends because everything inside the switch seem to be in good shape and the four contactors and springs operate properly.

Am I right that R1 gets current from the windshield washer switch just to run the wipers when the washer switch is engaged? Should this issue have anything to do with the missing high speed? I don't think so, but I might be wrong...