You are right on the money with the 5 cyl grenade scenario. That is exactly why Vovlo designed the system to include a flametrap. The turbo's don't have a flametrap - because in the case of a turbo, the backfire flame would have to pass through the intercooler and the turbocharger before it reached the hose "T" that leads to the oil separator. In the normally aspirated engines (non-turbo), the distance to the "T" is about 10 times shorter and there are no other components in the way. Personally, I would not remove it unless I saw something official from Volvo that said it needed to be removed. If you check/clean/replace it once a year like you say, and check the connecting hoses at the same time (and replace if they are getting plugged the flametrap should not be a problem.
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'95 940 Wagon, '85 240 Wagon (RIP), '91 740T Sedan