I'm just going to throw this out there, Rondo was the Celtics best player for long stretches of the postseason (if not the Finals) but four seasons in, there is no reason to believe he's going to develop an even mediocre shot, from the charity stripe or long range. He was worse this year than in his rookie season.
(And is there any precedent? A guard that was a sub-65% free throw shooter early in his career improving to 75-80%? I'd be more optimistic.)
As for the trade proposal, Rubio and Cousins (assuming he's available at 4) would add both more talent and more "What If?"s than a Marvel comic book.