Dexter killing the wrong man may have not been as integral to the narrative as I would have liked (the whole season is about Dexter slipping, but Dexter killing the wrong man would have been a big event any other season), but it still happened. I didn't see that you mentioned it (I was incorrect), which felt like an incomplete look at the season. That's why I brought it up. That the guy is a "pervy photographer" is essential to the storyline, though. Not to excuse Dexter's actions, but to demonstrate how quick Dexter is to judge and execute. All he sees is that the guy is a "pervy photographer" and that's justification enough. That the guy denies murdering the girls doesn't even matter to Dexter. It's a development that carries through two episodes, but given the show's focus on Dexter's internal struggle, it's a shame more isn't built on it, definitely.
It really isn't convincing that Lundy couldn't bring somebody onboard about Trinity, I agree. I can buy the "suicides" with the jumpers not being picked up by law enforcement, since they don't leave strong evidence of homicide and were often the site of suicides other times during the year, apparently. Lundy himself doesn't recognize the pattern with the kids, who are classified missing rather than murders...though if all Trinity is doing is burying them in concrete they're not going to go undiscovered for long (at least in real life). But that still leaves the women murdered in the bathtub and the men who were bludgeoned to death. There were certainly enough victims to establish a pattern, and having Frank Lundy endorse the theory (he apparently solved the Green River Killings before moving on to the Bay Harbor Butcher) one would think law enforcement would give it a serious look. Actually, this could have been tidied up by changing Trinity's M.O., leaving Lundy with a bunch of "suicides" that he thought were the work of a serial killer.
I'd agree that Anton wasn't that interesting. Not entirely unbelievable when it comes to Debra's character and her love life, but never very interesting. I'm glad that he was brought along after season three (rather than brushing yet another element of that season under the rug, like Rita's real estate job or Angel's new relationship), but he didn't do much then and didn't do much here, either. No loss when he left (and, thankfully, Debra didn't have a romantic subplot this season after that).
You claim that Dexter isn't about realism and I agree. But that also leads me to question why you're so unable to believe the way Dexter weasels his way out of the Mitchell's home. He flashes his laminate. He gets the hell out of there before he can be identified. Works for me. Season five is definitely following up on the Kyle Butler alias, which seems like enough for me, too.
I don't get what being old and jaded has to do with a couple of interesting plot twists. Trinity the presumed lone wolf having a family (just like Dexter!) was potato chips to you? That's the premise of the whole season! Arthur tracking down Dexter and finding out his true identity wasn't a chilling moment? Really?