One problem: having Yuki look exactly like Ranma seems to come out of the blue.
There's absolutely no reason for it. It's not fair to the reader to bring up
a "mystery" of why this woman looks like Ranma and then answer it with "maybe
it's this character who happens to look like Ranma by pure coincidence".
Also, if the Ranma cast knew Yuki in high school, then when an adult who looks
like Ranma is seen, there should be some people who say 'hey, could that be
Yuki?' They shouldn't all be saying 'hey, could that be Ranma?' while not
even *considering* the possibility that it's Yuki. The reader might not know
about Yuki yet at that point, but the characters do, and they should act like
it.