1999 Chevy Venture How can my heads be cracked/warped if it

This could get long. I have been very fortunate to NOT have many of the catastrophic disasters associated with the Venture. I actually quite like mine and was hoping to get 200K out of it. (Probably another year) Anyway, I was driving it all day, filled it up with gas and was sitting in traffic when it almost stalled. It didn't completely stall, but when the RPMs came back up, it smelled like sulfur and ran really rough. I drove it 1/2 a block and had it towed the next day to a dealer, which I'm thinking might have been my first mistake. It never over-heated, nor has it ever overheated in the 5 years I've owned it. The dealership suspected an intake gasket as they said there was anti-freeze coming from the exhaust. (My husband didn't notice any when he came to pick me up.) There was no white smoke.

The dealership then had the vehicle for 10 days. I got a call a week ago saying they were having trouble getting it back together and they did give me a loaner for the weekend. Then on Monday, I got a call saying that it was not the intake gasket, but they "suspect" it was the head gasket. (I replaced the head gasket about 2 years ago when we started losing anti-freeze and because of forums like this, avoided engine failure) He also mentioned a catalytic converter problem (which in my feeble little mind is the only thing that might be wrong) and said it would be $900 to replace. (I threw the BS flag on that one) Anyway, he also said that they suspected that since the head gasket MIGHT have blown that I would need to either replace or re-do the heads or put a used motor in it. Either of these solutions totalled close to $4,000.

Here is my question - if the car never over heated and was having NO issue before it stalled - how can my heads be warped and cracked. AND how would they even "suspect" this if they didn't test for it. I'm not mechanical genius, but isn't there a compression test or something? At any rate, I am beyond frustrated. I am hoping this is something much simpler. Maybe I'm just in denial.

I, of course, told them that I would be having the car towed to my mechanic for a second opinion. When I went to his shop to get some things out of it, he asked me to take a look under the hood. Half of the engine was in the BACK OF THE VAN - IN BOXES! Plus, they left the driver's side window down and because everything was unplugged, he couldn't roll it up. It sat outside overnight when we had torrential storms. After my husband (who is out of town, of course) called them yesterday, they came and towed it back and are going to put it back together so that my mechanic can run some tests himself.

Am I completely out of line for being suspicious of their diagnosis? If I have head damage, it is what it is, and we most likely won't fix it. If it's just a cat. Converter - I will be terribly upset!