She can still be safely spayed and handled by a competent vet, assuming she doesn't have any underlying diseases that would make anesthesia excessively risky. The vet will run pre-operative bloodwork to make this risk assessment.

As long as you explain your cat's aggressive behavior to the vet beforehand, the vet can take necessary precautions to avoid being injured by your cat.

Laurie

Is she yowling only when she's in heat or is it all the time? She could have something medical wrong that is causing her pain, or the yowling may be due to the fact she's blind. Fear, confusion and frustration could cause the meowing, but is there a chance she's also deaf?
I can understand her lashing out with bites and scratches if someone suddenly surprises her......and especially if she's deaf.