The Strawhats should all be working to be better, it's a constant struggle up the foodchain. A powerup shouldn't be uber new members. And if Jinbei joined it wouldn't just become the monster quartet, the power levels of the crew, which have been fairly constant for the whole series, would be totally thrown off. How can Luffy fight the main baddie when he's got the strongest fishman and a shichibukai on his crew?
I can see how your theory is plausible, the war is making things pretty unstable. But even if the shichibukai disband, Boa's position in terms of joining the Strawhats would be the same.
The shichibukai aren't as scary and untouchable as they used to be, sure, but for now, they're still a big threat. They're still part of the power balance who were given a special place in the upcoming war.
Whoa, I don't know about that! If each yonkou crew was that amazing they'd have taken over by now.
As for the last part of your post, no she's not an exact clone, but think of it superficially: if the entire crew of weirdos were lined up, it would be strange that two of them would be so similar. Sounds dumb, but if she were destined to join don't you think Boa would look a bit more unique?
I just think it would be something fresh to have some new monster in the crew. Brook and Franky are just middle-tier. And if Boa joins who will be there to fight Nami.