Looking at all the points raised, I think the only points against Rebecca that carry any weight are Soldier-san and her lack of any obvious reason to leave Dressrosa.
As for the others, her being a Nami look-alike isn't actually a reason she can't join, it's just a reason a number of people don't want her to join. There's really nothing to suggest Oda won't make her join for this reason. And her personality is a subjective point, which again makes it a reason people don't want her to join, not an actual reason she won't. The quirk thing is also in the minds of the readers. If she doesn't join it will be because of a bigger reason than the fact that she doesn't eat while standing on one leg or some trivial crap like that. I don't like the way perceived trends can apparently be used as evidence in this debate when they aren't stated anywhere by Oda or one of his characters.
So, that leaves the fact that she has no reason to leave and the fact that she has a reason to stay. The bigger of the two is obviously the latter, since her only stated desire is to live with Soldier-san. Ignoring Soldier-san (I'll get back to him) she actually has no reason to stay. A lot of people have brought up her princessness and her family, but while her ties to Dressrosa exist to other people, they don't to her. Everybody hates her and she hasn't seen her Aunt or Granddad for ten years. And her desire, as stated by her, is to live with Soldier-san. It's not 'Live with Soldier-san in Dressrosa' or 'live with my family' or whatever. Soldier-san is the only part of it that actually matters to her, so take him out of the equation and she personally doesn't have a reason to stay in Dressrosa.
So now let's talk about the possibility that Soldier-san will die. I know there are a lot of people who don't think he will because of Oda's past form when it comes to character deaths, but there are also those who are willing to entertain the notion on account of all of the times the idea has been brought up within the manga itself, as well as the allusions to the story of the steadfast tin soldier. If he does, then Rebecca - as of right now - has nothing tying her to anything. Not Dressrosa, not the Strawhats.
What decides her fate at that point is what happens during the rest of the arc, which is mostly unknowable. She'll either find a different reason to stay, or a reason to leave. Thus the conclusion I derive from all that is that whether she joins is down to Soldier-san's fate + how her character develops over the rest of the arc. All this stuff about what people think the Strawhats need in terms of appearance or gender or personality or strength or jobs is all just that: what people think. There's nothing in the manga or from Oda himself to support any of those thing as criterea for a Strawhat.
Personally I think there's good reason to believe that she will eat the mera mera no mi (which I won't go into now because this post is tl;dr enough as it is), and that given the right circumstances that would make for ample reason for her to leave Dressrosa and for Luffy to ask her to go with him. In the event that she does eat it therefore, I think it comes entirely down to Soldier-san's fate.