I don't think William or Charles will lose sight of the fact that, irrespective of Harry and his tantrums, the monarchy comes first.
Charles has the example of his mother to look to, plus he's waited his entire life to get to where he is now. I think he appreciates his time as King won't be long and that he doesn't have long to make his mark - he won't want to go down in history as the king who let his ludicrously entitled and out of touch son bring the whole edifice crashing down.
He famously doesn't like confrontation, but I also don't think he's a pushover. I think, like most people, he has a point beyond which he'll be pushed no further, and I think/hope Harry has tested Charles, in his role as king, if not as a father, to the limit,
Charles will also be mindful of the things Harry has said about Camilla I'm sure so will be very wary of allowing Harry back in any meaningful sense. Plus, he's apparently fond of William and must be able to see that the monarchy, in William's hands, will be fairly safe. Charles is reportedly extremely fond of Catherine, and of course his grandchildren, so I'd bet my house on him seeing the bigger picture and not entertaining any way back for Harry.
And of course, presumably Charles has excellent advisers around him, and any of them worth their salt must surely know that there's no way back for H and M. They're just so unpopular. They wouldn't give M the satisfaction of bringing the monarchy down (I'm sorry, I can't remember whose post I read this in now!) although I've no doubt she'd love that.
The only issue I can forsee is that if William decides he's had enough. He may see the strain that his father is under, he may be sick to the back teeth of his brother and his shenanigans, he may realise that his wife's health has deteriorated as a result of this mess etc, and just decide he's off! (I know it's nowhere as easy as that but you get the picture!). I don't think he'll do that but who knows ...