Definitely compression test to sort out head issue. The good news is an overheat will almost always, 99.99% stay confined to the head gasket on these tough motors. Let us know where you live ( pm me if you want to stay private) as we may be able to connect you to a good indie in your area. Upstate i...

First - STOP DRIVING THE CAR. You may cause further damage and ruin the engine. It sounds like your head gasket is blown. Best things to look at are the engine oil, it most likely will be milky brown and the engine coolant if any is in there, it may have oil mixed in. This is an almost 100% head gas...

Good reference for parts

Euro
FCP
IPD
links above

My second most all time favorite for getting OEM part numbers

http://volvopartslisle.com

(My favorite is pinging June and getting her receipts)

Between the moment when you adjusted the plug gap and was idling fine, and the moment it didn't start, was the car at the garage? Did the mechanic work on it? Also good to know that the plug gap is a very sensitive thing, really just slightly out of gap and it can go crazy. Shaking is serious misfir...

The throttle body (is that what TB refers to?) cleaning sounds a bit out of my realm. Maybe not, but have never done this. Unless you mean just squirting in that cleaner and revving. That I can do... It's easier than replacing the air filter. Just undo the screw clamp at the large hose on the throt...

Next steps are evaluation of fuel pressure at rail, while we still have the poor running condition present.