Sorry, but your bolded sentence made no sense whatsoever. Perhaps you're trying to say he would sign for the most he could and then a couple years later pull a Melo. But you always have to worry about that with any player you sign. All players could at least try to force a trade at any time. Second the Celts don't have to participate in sign and trades if they don't want to. If the player is trying to do a Lebron then the other team is the one paying him, not us.
Make your bolded sentences make a modicum of sense and we can have a conversation. I have never heard of a player ever signing in FA for top dollar and then turn around 45 seconds later and say "Now trade me" and of course even if Rondo tried to do that and he's as terrible as you say there'd be no market for him, and presumably his agent would know that. And further, even if he did that the Celts still wouldn't have to trade him if they didn't want to. The Mavs are well aware Rondo will be a free agent and they aren't concerned about any doomsday scenarios. Lebron's free agency didn't destroy the Heat or Cavs, and they would have been dumb to trade him before free agency in any case and not try to resign him.
No, it makes sense but let me try to help you through it. Let's start with your admission that
"It was clear Rondo was going to go to free agency. That's when we offer him more than anyone else..." Your words, not mine.
So your genius plan is to let Rondo go to FA. You offer him the highest salary of anyone in order to try to keep him. So you're assuming that:
1. No team would offer as much as the Celtics
even if they could. So the Celtics are the highest bidder and in essence are overpaying for his services.
2. If #1 is not correct and other teams would pay Rondo the max if they could, you then have to hope that he doesn't force a sign-and-trade (aka Lebron from Miami). If this were to happen, the Celtics would have a lot less leverage and the deal they receive would be considerably worse than the one they just got. In your imaginary world athletes may not do this, but in the NBA this happens quite often. Team A holds the Bird rights, Team B is willing to pay that salary, player decides to play for Team B and forces a trade from A to B where A gets very little in comparison.
3. Of course, this is also assuming that Rondo is driven by money and that he will take the highest deal rather than decide to leave to play for a contender anway.
4. Alternatively, there is a chance Rondo would stay and take a hometown discount. Of course, if this were the case it is extremely probable that he would have already been resigned. To believe this scenario, you must believe that Danny either hates Rondo or is so incompetent as to defy belief.
So overpay, get less, get nothing or irrationally hope for a miracle. Great plan. Fantastic!