Interesting idea. I like it. Sounds like it could lead to dozens of hilarious situations.
Some notes: mental springs dont seem to carry over to the original form. The warrior who felt into Penitent Monk spring was hyper-violent while not transformed. Rouge was a normal girl, so long as she didn't Asura up. Both seemed to have no love for their cursed form. For Ranma in this situation I think he would not mind the benefits of his cursed form, but there would still be no, so to say, leakage into his original form. He'd still be a loud-mouted martial arts nutjob. For laughs, have his mature-woman form be worried about becoming a "Christmas cake" and on the lookout for steady companionship. For even bigger laughs, go for the "young and virile". That would certainly freak Ranma out and have him trying to balance the "respect" benefit against the "OHGOD, I WANT TO KISS OTHER MEN!!" downside.
For the Tendo sisters, I think since Kasumi knew their mother the best she might resent an older woman trying to butt in to her harmonious family. A housemaker duel would be pretty hilarious, I think. If nothing else, spiteful little things Kasumi might do to drive away the invader would be pretty funny. On the other hand, Nabiki and Akane, particularly Akane, might rather enjoy having a mother figure around and, if you want to play it for laughs, have them constantly splash Ranma.
Soun might be a source of great hilarity if you make Ranma's advances unwanted advances. Like, say, he wants to remain faithul to his dearly departed wife and Ranma-baasan wants to jump his bones. Having Nabiki and Akane support "her" advances, while Kasumi and original-Ranma trying their best to ruin them could lead to some pretty funny situations.
Genma would be funny, but in a different manner. Idea here would be that Ranma looks like Nodoka's sister, right? or a slightly younger Nodoka? Well, have him freak out over the change and Ranma-baasan calling gim "Genma-san" in the same tone Nodoka used to. The most hilarious reaction here would, of course, be full blown fear.
I don't think "Secretary" would be a good turn, however. It would require the introduction of many new characters, and take the focus away from the highschool element. Go for the "hot for teacher" route. That way you can keep interactions with Akane and Nabiki at school, even Soun during PTA meetings.
The Nodoka interaction could be either very dramatic, or purely for fun. You've already outlined the comedic approach in your original post. Drawing it out, as was suggested by others, would be very dramatic. Dramatic approach could be use as a wrap-up arc for the story, I think.
Overall, it reminds of WFROSE's "Older" but with an inverted apprach. A very interesting take on the "age" curse and one I would be willing to read.





