Barca tried chasing him last year and *almost* got him so I reckon he's gonna leave this summer. There's no point keeping an unhappy player at your club.
He'll get gametime and a lot too, you don't splash 40+mil for a benchwarmer, no not even Barca are that stupid. Plus, Xavi welcomed the extra competition anyway.
I don't think it's really betraying Arsenal, he joined them originally as a kid to get game time and all the fans know that Barca was his dream club for years anyway so if he wants to go then let him.
I reckon your proposal is also good considering by that time, Xavi won't be a regular anymore (too old) and Cesc can slot in easily. I remember seeing a goal article questioning whether Barca need him or not, considering they had Thiago who's pretty good imo.
My two cents.