I can't see H & M ever being accepted back into the royal fold. The absolute chaos they created, the reputational damage they caused, the conflict and divisiveness that dogged their every footsteps. Not happening imho.
I'm also not sure that Charles is subbing them. I can well imagine there being provisos for Lily and Archie, but not so much for H & M. Didn't he say something to the Queen about not taking H's calls because he wasn't a bank? I think Charles is one of those easy going, genial, bimbling types who is very conflict averse - but who does have a limit, and when that limit is finally reached, that's it.
Apart from William and Charles, there are some pretty strong and forthright personalities in the royal family. If, and it's a massive if, they came back, I can't imagine some of those people holding their tongues. (In fact, I imagine some of them would love to let rip!) And one whiff of someone not being absolutely floored by the awesomeness that is M, would have the pair of them (H & M) straight back into tantrum/foot-stamping mode.
I also can't see them being accepted by the public. They themselves probably think they can win the masses back just by being their own fabulous selves, but I don't think it would take long before they were disabused of this notion. And then - guess what? Yup, back into tantrum/foot-stamping mode.
I can see that Harry might want to come back for various reasons. But Meghan - not in this lifetime thank you very much. I don't think she's brave enough to face up to the RF after all that she's said and done - you only have to look at her demeanour when she and H accompanied William and Kate on that walkabout after the Queen died.