1997 Pontiac Grand Prix intake manifold repair caused blown

I had the intake manifold replaced on my car at the garages suggestion. After getting the car back it was not running right. Idle was inconsistant, and downshifting too much. I had to drive the car for 3 days before I could get it back to the garage, by that time the engine was knocking sounded like the valve train was rattling. The garage tried to place all the blame for the problem on me saying I waited to long to have the gasket replaced, but the car was running fine before they did the work. I ended up taking the car to another garage where they found that the previous garage left the o-ring off of the pcv valve in the intake manifold. They also found that the rod bearings were spun. Now I am fighting with the original garage to have them pay for repairs but they are denying any responsibility. I am understanding that a vacuum leak like this could cause pre-ignition which would account for the damage. What do you think?