The QO is the first tangible signal. They could still sign and trade Grant (and potentially use the MLE), but I doubt that they would rescind it.
My guess is that there is not going to be that much money out there for Grant. The landscape has changed plus I think his value dropped back down a little. $10M+ is a pretty big offer for Grant in this climate in my mind, and the Celtics would match that. I don't see anyone offering enough that the Celtics wouldn't match.
I am going to predict he signs a short deal at or near the QO, with some player favorable opt outs or whatever they are, so he can get back into the market when he wants to and hopefully have a more favorable landscape.