This is an interesting one....

As Klaus says, time to step back and ponder. Let's recapture your actions. The way I understand it, you originally had the cam sprockets on their marks and the belt still in place. You wanted to loosen the crank bolt but had no way to counter the CCW crank rotation when loosening it. So you said "I'll lock the cams in place and the belt will keep everything static while I crack the bolt loose." So you went to put the cam tool on and realized the slots were about 10 deg off of parallel.

Q.) Were they BOTH off of parallel?
Q.) Do you have VVT on BOTH cams?

Anyway, you then rotated the crank and cams together (belt still in place) until the slots were parallel.

Q.) Do you recall which direction you rotated everything to achieve parallel slots?

So you then got the cams locked and put the heavy torque on the bolt CCW. Then "slip" went the crank sprocket.

Q.) Am I on track?

On a 5 cylinder engine, a piston reaches the top of it's stroke every 72 degrees of crank rotation. I'm going to surmise that Volvo placed the index marks on the sprockets at roughly 36 deg either BTDC or ATDC - that would put the two pistons that are closest to top-of-stroke EQUADISTANT FROM the top-of-stroke, and both pistons would be out of harms way of any possible piston/valve interference. So when you tried to loosen the crank bolt and the belt slipped, you may have had to turn the crank quite a ways before a piston contacts a valve.

Q.) Do you have a rough estimate as to how many degrees the crank slipped when you tried to loosen the bolt?

Something tells me you may have very well dodged a bullet on this one. If it were me in your shoes, I'd probably a.) leave the belt in place for the moment; b.) remove the spark plugs; c.) CAREFULLY bring the crank around to its index mark; d.) pull the belt off; e.) put the cam sprockets on their marks; f.) put the belt back on with proper tension; g.) rotate everything around a minimum of two crankshaft revolutions to very no interference; h.) perform a compression check.

Hopefully you have NOT loosened the cam sprocket bolts yet.

Good luck!
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the ignition - now back to the brakes again)