Late to the party, just got caught up.
While I find the twist ending interesting, I'm not pleased with the road they took to get there.
1) Dexter went to the arcade after getting a call from Trinity and then didn't figure out it might have been a setup, or notice Trinity's van following him back to police HQ? I don't buy it. Implausibility factor, 5/10
2) Dexter unable to maintain his cool (how many times has he ever gotten out of control angry?) and getting arrested with Trinity knocked out in a van 20 yards away and knowing who Dexter really is? I don't buy it. Implausibility factor: 8/10
3) Rita forgetting her ID and having to return home just in time to run into Trinity? I don't buy it. Implausibility factor: 10/10
Too many coincidences had to occur exactly as needed in order to bring about the twist ending. They want to kill her off, fine, but then they shouldn't have sent her and the kid to fly somewhere on a plane and then magically had her miss a flight. Lazy writing. I also found the Laguerta/Batista sideplot an unnecessary timefiller.
Great show though and Hall brings it every season.