Well he talks on the other thread about his coolant light going on, the coolant gauge being all the way up, and his coolant hose being drained flat.
So yea, your head gasket probably cracked/blew and allowed coolant to pour into one of your cylinders. Since water can not be compressed very easily it probably put too much pressure into the cylinder, detonated, cracked a piston. Piston flopped around hit the head and scored the cylinder.
I guess the only real good news is that you couldnt have done much more damage. Maybe a bent oil pan and a bruised crank.
Also, its a some what known problem for our idlers and tensioner pulleys to break. I havent heard of them bending before, but definitely breaking. Which tends to leave people stranded, but if you keep driving on it then what happened to you could happen to anyone. If you are ever driving around again and the coolant temp gauge starts nearing the top, pull over and turn the car off immediately. Even if your late somewhere and only only a block away. Its better to be late than to be need a new engine.
Anyways, good luck with the rebuild. And make sure the mechanic uses vaseline with his bill.