I love your breakdown. Since this is all speculative, I'd like to point out a potential (albeit, small) flaw in your logic. Jason takes out assorted, hired guns/toughs and trained security personnel throughout the films. More often than not, he makes this appear pretty easy.
Why wouldn't we assume that these people are a few steps above a lowly commoner. Like the chump journalist that eats it in the 3rd movie for example, as we see how poorly he fairs against the same odds as Jason. If that were the case, Jason seems to effortlessly dupe and/or b!@ch slap mobs of these guys. That would ostensibly give them at least a level or two in fighter or a similar martial class, since there has to be some delineation between a trained fighter and a commoner.
The fact Jason is able to wreck droves of these types of 'characters', I would say is pretty strong evidence that he is of higher level than just 5.
Again, I feel your analysis is pretty spot-on, just playing Devil's Advocate!
I think a couple of brawler levels would work fine, maybe give him hero points on top of that.
I agree with you that the people he fights don't need to be commoners. I think a level of warrior would work. A warrior with combat related feats is a big difference from a commoner with non-combat feats.
I don't think it's a good idea to give him anymore than 3 levels. Making him level 6+ starts to put his power level in the same range as large sized drakes and 16-ft. tall elementals.