Mechanic's Fault?

Hi everyone, I’m new here.

I’m not sure what to make of this situation so I though I would put it out there for some opinions/suggestions. Here goes.

I noticed the brakes on my 92 Buick Lesabre were getting awfully loose(aka had to put the break pedal to the floor to stop). I just moved to a new state about a month ago so I had to find a new mechanic. I brought it to a local shop for a look. They said the rear drum brakes were shot and needed replacing. OK, couple hundred bucks, no problem go ahead and do it. Get a call the next day that the rotors are warped and can’t be turned, they need to be replaced, and one of the drums was particularly difficult to remove and upon successful removal the speed sensor was so brittle it just broke. Car will also need a new speed sensor (another $200). I have them do the work and now they are into me for over $600. OK, so I get the car back and it brakes well, only now the ABS warning light is lit on the dash. Mechanic guy “assumed” it had been on previously and when he checked the error it was reporting from all points of the ABS system. (by the way this was a AAA approved mechanic).

I have two questions:

1) what the heck did they do to my car?

2) When removing the drum on the “difficult” side, was it neglegence that broke the speed sensor? I was reading about replacing the rear brakes in the Haynes manual for my car and it urges caution if the drum is difficult to remove and warns not to use excessive force then lists several steps to remove it if it is difficult.

Sorry for the length here. Hope you all can help.