I've never been a fan of the kicked out of super special place ending either.
"Thank you for saving us. Now get out."
"But I've made all these great friends and I really like it here."
"Can I visit on the weekends?"
"How about once a month?"
"Once a year? Please. I'm begging you here."
"Get out now. Back to your world with you foreigner!"Um... yay? This is good how?
On the plus side, the kids got to stay for quite a while in Narnia, on the down side they got to stay for quite a while in Narnia. It was their home for fuck's sake. They barely even remembered their lives before. Even if Narnia does operate on a principle of needing to get the illegals out before they can have anchor babies, don't you think an exception could be made for the royalty who fracking grew up there? Pass the Narnia, "England is but a Dream to me" Act.
You can't just cast people out of their home and have it be a happy ending.
They should have ruled for a little while, left the wardrobe, come back to rule for a little while, left the wardrobe, and so on. They could have stopped by every day. Rule as long as it takes to get stuff in order, prepare Narnia for their departure, and then go back out of the wardrobe. There would be continuity of government and identity. Their Narnia selves would never get too much older than their England selves because they'd never stay in Narnia for all that long. Narnia would be structured around the idea that there would regular periods with rulers present, and regular periods with rulers absent. They'd have to deal with about an day earth time without their royalty, but after that the royalty would be back to deal with any problems that cropped up.
The reign of the kings and queens of Narnia could thus endure for thousands of years with a fair degree of stability.
[Original Work Index]
As for the second, larger, part about Narnia, I haven't done enough Narnia stuff for there to be an index for it yet.