Ryoga's curse is much worse than simply an inability to tell the cardinal directions apart or follow directions. In his introduction arc he literally walks a thousand miles in one week. Assuming he stops for at least 8 hours a day to sleep, eat, bathe, practice martial arts, ect. (and thats pushing it) that means he spent 112 hours total walking for that trip. That comes out to an average walking speed of almost 9 miles per hour. While that speed is within normal human speedwalking limits (barely), I doubt even Ryoga could keep it up for 1000 miles . And that's assuming he went in a straight line, which is
extremely unlikely. So either Ryoga can walk at ridiculous speeds for insane lengths of time, or he has some kind of supernatural means of getting around. Considering that he regularly travels to Hokkaido (which is a separate island) and has once even traveled to
Hong Kong (which is in China), without ever crossing a body of water, I'm inclined to believe the later.
My theory is that he has some kind of hereditary magical curse that causes him to teleport great distances without realizing it. That is about the only way to logically explain
this map. The curse worsens as the Hibiki ages, which explains how Ryoga was able to survive to adulthood and was able to (irregularly) attend school until Junior High. Also, it only activates when the Hibiki is completely alone, a good distance away from any civilization, which is why he never vanishes right in front of anyone. And because I don't want to think about how both of Ryoga's parents have it if it's genetic, I'm going to assume that the curse magically extends to spouses.
Therefore I think that Kasumi does have the curse. However, due to her rarely leaving the house, she knows it's layout intimately and can get around it without getting too lost (she spends an hour or so a day lost in a broom closet, but that's the worst of it). Also, I'd imagine that the Tendo home is probably filled with neon arrows running along the floors and walls to help the family get around, and of course Ryoga's dog, Shirokiro, and her puppies live with them and guides them around. So the curse isn't much of a handicap for Kasumi. If she does leave the house, she just takes one of the Hibiki family dogs with her.
I agree with your points about Jusenkyo. I like the place, I think the curses are a landmine of comedy potential, I just don't see much opportunity to use them in this fic. The only characters I could see being cursed are maybe a yokai or fellow Amazon that followed Yemon to Japan, Taro's kid, of course, and maybe a few of the challenge-of-the-week martial artist antagonist-types that wander through town. What about Mousse and Ryoga though? Did either of them ever get cured?
I don't get what your saying about Kasumi affecting the curse in her child... are you saying that she somehow manipulated the genes her daughter inherited through sheer force of will? That's pretty freakin' amazing! And rather difficult to believe.
I'd keep the Hibiki daughter's curse just as powerful as Ryoga's was at that age. You can write her as being lost a lot less simply because she's a lot smarter about it than he was. She carries a cell phone at all times and knows most of the cab drivers in and near the tokyo area on a first name basis!
I don't think having the Neo-Hibiki know any hidden weapons is really necessary. Ryoga could carry just about everything under the sun (including a tent, kettle, and a few dozen sets of clothing) in his enormous backpack, and he would teach his daughter that skill. They're just really, really good at packing.
I'm pretty sure she would use an umbrella for a weapon. It's a very practical item for someone who often gets lost in the wilderness, and might not be able to find shelter when it rains. I don't know about that "courtesan/geisha" style you mentioned though - Ryoga never 'twirled his umbrella in hypnotic patterns' or used 'exaggerated coy/flirtatious behavior' (that sure is one funny mental image though). He pretty much swung the thing like a warhammer, and considering that the umbrella weighted a few hundred pounds, that's about the only fighting style I imagine one could use it for.
The idea of her learning the basics of a great many techniques, but never mastering more than one or two is pretty good... but do you really want to invent that many original techniques? That's a lot of work. If you can do it though, than go for it. You could easily say a few of her techniques were taught to her by Soun, justifying them by saying that Soun never used them in canon because he almost never fought anyone, and he didn't teach them to Akane because she wasn't advanced enough or he didn't trust her temper. I think the Hibiki-daughter would at least be very proficient in her family's Iron Cloth techniques though.
I still don't think Happosai would teach her anything. He's pretty sexist. He did teach Hinako and Natsume and Kurumi, but they were exceptions because they were very unfortunate kids and triggered his weakness to children. He taught Hinakoto save her life and Natsume and Kurumi because they were orphans and he wanted to give them a goal to give them hope. The Hibiki-daughter has had too good of a life to trigger his generosity, and I doubt Ryoga or Kasumi would give the old pervert permission to teach their child anyway. I don't think Happosai would teach Ryoga either, simply because the two never got along in canon, and, like Ranma, Ryoga wouldn't be willing to put up with Happosai's perverted training methods.