I know what you mean about windows of opportunity, how did Forest Gump put it, life is like a wood between the worlds, or something like that.
Any who, I do not think it is particularly likely that Susan opens the box. Since it contains the effects of her dead parents and siblings it could be difficult possibly even painful to go through. Not to mention that it represents a part of her life she apparently already wants to put behind her. Indeed Susan opening the box, though perhaps not a sine of full repentance, is at least an indication that there is a spark of the real Susan left, one that she could chose to extinguish if she wanted to.
Oh, and Aslan did not tell Digory to destroy the rings (he must be thinking of Gandalf, very similar to Aslan in most respects.) Aslan in fact told Digory to bury the rings so that no one could use them again, which he did not do either. So its Digory 0 Frodo, well we have to give him an A for effort any way.