Interesting question @LadyMuckingabout
Just my opinion but I don't think for one minute they will come back to a Royal Estate here. Not for one minute.
What circumstances could they come back in? There aren't any AFAIC. As far as SF.. she was not given as much as she wanted or needed in the divorce settlement, she had hugely extravagent tastes and was bordering on bankruptcy more than once. So she came back. (just my opinion!)
As time rolls by and their life in the US becomes more firmly established, it would be a huge wrench to move children back here. There would be no role for them that I could see. And the media would go wild and who would want to live with that hanging over them day in day out? It's bad enough when they live thousands of miles away.
I think Harry has been a square peg in a round hole and not happy within the RF for a long time, then along comes Meghan and there was a reason to make the break.
Who knows on their finances, that's up to them isn't it...
So - to me - bottom line, can't see it, no.
Thanks for the question, I like discussion!