I overall liked the last episode.
Isaak is great. I love this character (probably because I'm a fan of the series "Rome") and enjoyed his "gay turn". It's one of the best things they could've done with him. It gives his character much more sympathy, which is THE way to go. Personally, I would wish for Isaak surviving and just leaving country, or even continuing on the show. But even when Dexter kills him (which I count on), it will FINALLY be another kill which is kinda tragic. At least for me. The last few seasons lacked this subtlety.
Regarding Debra admitting her love, I found that scene pretty realistic for two emotionally "handicapped" people. I never liked the thought of Debra falling in love with Dexter though. And I like Dexters relationship with Hannah, but her being a real murderer (not just people who seriously wronged her) can only ultimately result in her death. Although I can see Debra doing it herself...
What still bothers me is that she's doing all this investigating on her own. At some point her boss (or her bosses boss) will go, "Hey, why is that Captain out checking on leads when we have guys like Quinn hanging around the station?" People at her rank just don't do that, they don't have the time, or the need. They can just tell people to look into things, and report only to her. It would look less suspicious than whatever she's doing now.
It makes sense, that she does it on her own. When you're so high in the hierarchy, you have to protect your appearance. If she follows up on this already solved case, her partial situation (friend/former lover of Doakes) could endanger her political status. If she involves her employees, this risk multiplies. Therefore she can only use help from people she can trust (to keep their mouth shut) until she has proof.
I see your point, that a person in her position usually has too much to do elsewhere. But, hey, don't be picky! On the realistic side, she's extremely slow with her investigation.
On the other hand LaGuerta has always been one of the less brightest bulb in the department. ;-)