If you have a timing light (or if not, borrow one), connect it up before starting the cold engine. Watch it while cranking the engine. Of course if the engine starts, you'll have to wait and do the test again later. If it doesn't start, did the light flash? At least you can maybe determine if it's a too much fuel or not enough ignition problem.

Your mechanic should have (and maybe they did) check the fuel pressure before changing the regulator. Do they specialize in Volvos, or do they "Specialize in All Makes and Models of Domestic and Foreign Autos and Trucks"? If the latter, post your location and possibly someone can suggest a good shop.