Good point. I think Aslan told Digory what would have happened in those three instances you mention because Digory resisted both his own temptation to try the apple and what Jadis was tempting him to do. Even though he had originally approached Aslan looking for healing for his mother. One of the reasons why Jadis could tempt him at all, was because Aslan had not agreed immediately to help his mother, and demanded that Digory should do something to rectify what he had done.

Digory and Polly were already in Narnia, along with Jadis, as a consequence of visiting Charn, and Digory's folly there, in ringing that bell on the stand in the Hall of statues. Maybe to an extent, Digory and Polly, like Jill, in particular, needed to learn that there is a right way and a wrong way of doing things, and that wrong choices can bring dire consequences, not only on a personal level but also on a global scale.

This is also similar to Lucy being told "nobody is told what would have happened" because, like Digory in Charn, she gave into temptation to say that spell to spy on her friend in the railway carriage, after being also tempted to try the beauty spell.
The only one I didn't really "get" was the PC instance, which, if Lucy was to get her brothers and sister to follow her, would have required her to stand up to them a bit better than what she was able to do.

Personally, my theory on this has always been that Aslan wasn't telling him what would have happened in the sense of telling him how the future would be different if the past had been altered. He's explaining the rules of magic - specifically, the apples - in Narnia. At that point, it's still a real possibility that Digory could steal one of the apples from the new tree to take home to his mother, or to eat for himself, and the consequences that Aslan describes can still happen. I think Aslan wanted to clarify for Digory why, despite how difficult the choice was, Digory had ultimately made the right decision by bringing the apple back to Aslan instead of keeping it for himself - and to set the stage for giving him the good news that he is being given an apple for his mother.

On the other hand, when Aslan is speaking to Lucy in PC and VDT, in both cases she's wondering about how things would have been had she made different choices in the past. In both of these cases, it was too late to make the other decision, and Aslan would not tell her what would have happened if she had. I suspect that this also is why Aslan does not confirm or deny Jill's suspicion in TSC* that, if she hadn't pushed Eustace over the cliff, that they would've heard the signs together (which, in turn, leads to the whole book taking an utterly different direction).

(*I think this actually happens in the book. I'm vividly remembering it in the FotF production, but I don't have my copy of the book handy and I can't remember if it actually happens in the book.)

Basically, I think what Aslan meant by "No one is ever told what would have happened" is that, once a possible future is closed off by one's choices, you are not allowed to know what that would have been. If the choice is still possible - either for you or for another person - then Aslan might explain the consequences of that possible choice, if he is so inclined.

In both of these cases, it was too late to make the other decision, and Aslan would not tell her what would have happened if she had. I suspect that this also is why Aslan does not confirm or deny Jill's suspicion in TSC* that, if she hadn't pushed Eustace over the cliff, that they would've heard the signs together (which, in turn, leads to the whole book taking an utterly different direction).

(*I think this actually happens in the book. I'm vividly remembering it in the FotF production, but I don't have my copy of the book handy and I can't remember if it actually happens in the book.)

In Chapter 2, Aslan tells Jill that he must send her to Narnia at once, she has no time to spare - and Jill remembers that this is her own fault.

"If I hadn't made such a fool of myself, Scrubb and I would have been going together. And he'd have heard all the instructions as well as me," she thought.

For one thing, Jill doesn't say this to Aslan in the book, so a comment from Aslan would have been less natural. But he might still have commented on it if he had wanted to - and one reason why he didn't, might have been just what you mentioned, that there isn't an option to make a choice any more.

I think it's reasonable to think, like you do, that it is in those situations where the choice is still open, that Aslan gives advice by pointing out the different consequences of the choice.

I agree with Ryadian. Aslan in MN is just explaining the magic of the apples, that they do not work happily for anyone who uses them by their own will. Once he understood that, Diggory could possibly have worked out the consequences himself.
In PC and VDT by contrast, there's no way Lucy could have predicted what would have happened if she made a different choice, because that would depend on the choices others would then have made. That's something only Aslan could know.