Timing Belt Put On wrong

[Angermanagement]

I recently had my 1997 Mitsubitshi Galant repaired at a shop.
First my water pump needed to be replace and they told me I needed a balancer belt. I paid for the repairs got my car back and it drove like a champ for about 3 days. I noticed that it would start to stall when i were in stop eg. red light. or in park.
I had to drive with 2 feet (automatic car).
I took my car back to the shop they told me that it was my injectors that were bad. #3 was bad and #1 was weak.
The mechanic took out a spark plug and put another one in. Idled my car up so it would not cut off until they could run a compression check.

My car broke down completely 2 days later and would not crank. The first mechanice that took a look at it said that whoever put my timing belt put it on wrong. He took out the spark plugs and one was ate up (the part where the glass is) all the way to the grooves that screw it down.
I had it towed back to the shop who did the water pump repair and they told me that i "dropped a valve". Only one piston was beat up real bad and could not be repaired. He told me that if they put the timing belt on all of the pistons would be beat up and not just one?

My question is if the timing belt was put on the wrong way or the mechanic messed up something would this be a possible result or what he saying has merrit?

thanks

[dev3768]

my motor wouldnt even run with the timing off, i destroyed everything in the head and prolly damaged the pistons, its more likely a valve went in a cylinder, especially since you were driving the car and you never said you had the timing belt change recently, either way u need a new motor