There are filler episodes and then there are... filler seasons.
Just utterly terrible season. I was hushing the complainers after the first 2-3 episodes, expecting it to pay off towards the end. No redemption. I won't be re-watching this. Kind of blessing in way, though, it was so bad and so little worthwhile happened that one can safely just skip this season.
Got to rewatch s1 and s2 at some point because my memory of s1 in particular is very faint, but I rank them 2=1>>4>>>3>>>>5.
Re: Deb knowing Dexter's involvement. Upon first watch I was sure she didn't know anything, but after the comments here I rewatched the two key scenes again (Deb's comments). She is kind of enigmatic and has this weird look and face. About it "being all over now", perhaps she assumes that Dex was only helping Lumen (or whoever she suspected #13 to be) in this project but now that all the men involved in the barrel murders are dead (missing), she assumes that there's no killing left to be done for Dexter and she's therefore letting it slide. It was extremely subtle though.
Quinn's case is sketchy but I guess there's no way Quinn doesn't know about Dex, at least now that he falsely cleared Quinn's bloodwork even though Dex hates him. Dex probably realized that Quinn was sure about Dexter only after Liddy's death (Quinn still has circumstantial evidence at best against Dexter) and now he bought Quinn's silence by clearing him off Liddy's murder (which, as stated, was ridiculous). Btw. wouldn't Quinn's bloodwork in reality have been done by some other people than his own colleague because of conflict of interest? Just so much wrong in the finale, let alone the whole season.
But the good thing is that now that Lumen is gone and everything is more or less the same as when the season started, s6 has an easy task on improving from this season.