Thanks for trying to help these guys Sega Chief.
Umm...yeah I don't really know why those things would be happening. Lost Wing is more of a novelty than anything else. While I do think it is a lot of fun and a welcome flavor change from the normal VII, this mod has countless bugs, glitches, and errors and I will most likely never be able to correct them - especially since I haven't done any FF modding in years now. But if it won't let you get Yuffie the normal way, I'd suggest using Black Chocobo to alter your save file. Alternatively you could Gameshark her into your party, but I don't think that certain storyline flags will trigger (meaning you may be unable to do the Wutai quests), so just go with the save file editor. Other than that, I apologize for any frustrating issues with the mod, hope you enjoy it though.
@Hitachi There is no such spell called "regain" in my mod. Do you mean "Regen"? I am unaware of any problems with this spell, and no one else has reported that issue. Likely its not due to the Lost Wing mod. I do know some people have experienced trouble with "White Wind" though.