I can understand why you waited to pull over, Chicago and road construction just don't mix.
You now have a very iffy situation. The mechanic said that the compression was OK on all 5 cyl - head gasket OK. Did that same mechanic install the thermostat? The Volvo OEM usually fail in the open position, while aftermarket cheapies fail in the closed position. Find out what he put in there this time, and if it not OEM, have him replace it with a good thermostat.

Now you need to wait. Watch the fluid level in the expansion tank for a week or so to make sure it doesn't go down even a little bit. Check the oil, which you changed, to make sure it doesn't get muddy or cloudy. Check the tranny fluid for the same. Look under the car every day for even a drop of antifreeze, especially at the passenger side where the water pump is - you might want to pull off the cover over the timeing belt to take a better look at the water pump to ensure it is not leaking.
Damage has been done, but to what extent you will not know until you drive it for a while.
Get on Craigslist and start looking for 1998 S70s?

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)