Could you say "Vi devas sxin atendi." or would that mean "You need her to wait."?
Yes, you can. It is already reported.
Then how would you say "You need her to wait?"
Probably something like "Vi bezonas ke ŝi atendas?"
Just got to the imperative case, where they explain it. It would be "Vi bezonas ke sxi atendu."
Kial la kurso uzas "al"-n?
Kie ĝi uzas tion?
Ili ne plu uzas ĝin.
Why is vi havas atendi sin accepted?
It definitely shouldn't be. If it is then please report it as wrong.