Not really sure what the answer is to all your questions, but in short I think I make the call on the river as well for a lot of the same reasons that you did.

What value is he really representing on the turned 5? 55? Which doesn't really make sense either because would he really call 55 on the flop? Like you said, I think if he has JJ, 99, 44 or J9, he would be raising those on the flop. There are so many value hands that you can have on that flop as well as draws that he can get value from, as well as protection. There are so many bad cards for him. Any K, Q, T, 8, spade and so on. So again it wouldn't make sense to slow play his strong made hands on that flop, not that he couldn't, I just don't think he would. So again he is basically representing 55, but there are so many gutshots and other draws to bluff with on that turn card.

I personally don't think he has trips either. What Jx is he raising on the turn. IDK to me it just seems like he has so many bluffs and not that much value the way it was played, so I think I make the call. KQ makes a ton of sense. Fairly easy call on the flop with the gutshot, but doesn't have odds to call a second barrel so turns it into a bluff with the gutshot and 2 overs that block some of your value hands.

River bet is only half pot as well, so you only need to be right 25% of the time. And yeah I guess your blockers are kind of meh. You do block some of his bluffs and not really any of his value, but again what is his value? His line just doesn't make much sense.