Reading your post, I was starting to wonder how you've stuck with it so long. The scenes between Elba and Wilson are my favorite because they bring out Luther's Dexter tendencies. It's what I think Neil Cross is going for. A cop driven so dark he has serial killer qualities. Even though Cross builds in a lot of latitude by not explaining what got Luther into such deep trouble before Season 1, he crosses into unbelievable territory many a time.
The scene that I think is the biggest betrayal of staying true to Luther's character is during 1:2 when Luther is forced into Russian Roulette. He turns all soft and beggy. I think it's meant to show Luther's limit, but it reminded me of early James Woods characters who, when in a similar situation, say "f' it" and encourage the shooter to bring on the bullet. The Luther we meet during the rest of the show would do the same.
I don't know what your problem is with Ruth Wilson (and this isn't just my love of redheads). She's sometimes a bit of the cartoon Hannibal Lecter became, but more often a nice blend of Dexter and Moriarty with a large helping of Angel from Buffy the Vampire Slayer.