It appears that the problem might not be with rear brakes after all, my mechanic drove it around some more, pushed in clutch and slowed down car with emergency brake, no pulsating, no grabbing. He tried some other tests also, and came to conclusion that rear brakes were fine.

The car has not had any sort of alignment yet, no shims installed, no adjustment with camber/ caster/ etc...

If all these settings are way off (which is highly likely!!) could this possibly in some way contribute to an extreme grabby feeling when brakes are applied? The car is eventually stopping,

Is it possible that the booster or dual master cylinder could be contributing to problem?

I really want to get this car on road, extremely dangerous to drive at moment, but next destination is alignment shop