Sorry to resurrect a dead thread, but it might be useful.
I've just repaired an Amiga 4000 for someone who had the same problem (RMB always pressed, so early start-up appears with just LMB, and Workbench not reading RMB at all).
The fault was a capacitor, but not one of the usual suspects - it was C412, a 0.047uF polyester(?) capacitor. It was discoloured on one end where it had leaked or something. Once I'd replaced that, the 4000 worked perfectly again.
On the 600 it'd be in the same place, between GND and POT_0Y (pin 36) of Paula. On my machine it had a resistance of just 80Ohms, so the line was held low permanently.
Hope that helps someone with similar problems!