Being a Volvo tech myself who's seen both bad FPRs and hydrolocked engines (not on the same cars however), I'm trying to understand how this "bent valve" could happen from your description. A bad FPR either leaks gas through the diaphram into the vacuum line or causes excessive fuel pressure. In either case, the engine burns the mixture without hydrolocking. Hydrolocking can occur if an injector sticks opened causing a cylinder to fill with gas and I've seen that too BUT it doesn't bend a valve, if bad enough it'll bend a connecting rod.
A valve doesn't just bend by itself or by a hydrolocked engine.
I'd get another professional opinion before trying any engine work yourself as that's not an engine for an amateur (not simple like the good ol' 4 cyl red engines in comparison). BTW, it's alot easier to remove the oil pan than it is to pull the cylinder head BUT without engine experience, I highly doubt that you'll see physical damage by just removing the oil pan.
If you're determined to do your own work on that engine if need be, don't try it w/o getting the OEM engine overhaul manual (800-25-volvo) and be aware that costly special tools are also required to properly R&R the cylinder head. Having done that before several times myself, I wouldn't ever consider trying that job w/o the special tools.