(1) It depends what kind of jeans they are. Some are made with softer material and just feel good. (2) For girls who wear thongs/g-strings, the whole "riding up" thing is not new...It's not something a girl would be able to do daily.
ive never gone without underwear in tight jeans, but ill wear something that shouldn't be considered underwear. id imagine its uncomfortable for several reasons. but I go without underwear in tight dresses