I don't think she wants a "comeback". Technically speaking she hasn't gone anywhere so there's no comeback to make...
She seems happy in Vegas and when she's into it, she has some really good moves going again (see
this video) and the whole residency thing is proving to be a huge success (every show is selling out). There are some shows where it's obvious she is half-arsing it but off days happen to everyone.
Musically, Femme Fatale was a pretty solid album and spawned the biggest radio hits of her career and a successful tour on the back of 2 mediocre promotional performances.
Scream and Shout was huge. Sure it wasn't a great track but I don't think it would have been anywhere near as successful as it was were it not for her being on it - will.i.am's other collaborations - with bigger current stars - have failed to reach the same levels of success.
Britney Jean on the other hand is just... not great. There are some gems but overall it's a lacklustre album (personally I put it down to the involvement of will.i.am - I think they saw how big Scream and Shout was and thought they could replicate it. They failed). They hyped it as a Blackout 2 and the direction was changed.
It's clear Britney has very little say in what happens for her career: she wanted Alien to be a single, that has yet to happen and it's looking unlikely that there will be any further singles from Britney Jean. The Perfume video was destroyed (when controversy would have been a good thing). All the people she said she was working with bar will.i.am didn't make the album... etc
I think we'll see her take a long break after her Vegas contract is up. Her musical contract was over with the release of her last album so we'll see if she signs another or not. A lot of people just want her to be happy so even if that's not making music that's what matters.