I guess it's good that both are accepted, because saying "who" in place of "whom" is a common mistake. But in this case, only "whom" is actually correct.
There are some situations in which the course really needs to strike a balance between what is "technically correct" but rarely actually used, and what is "technically incorrect" but almost always used. This is one of 'em! :-)
Wait on is a US regionalism and therefore wait for should be preferred here.
Be that as it may, I really like this song: https://www.youtube.com/watch?v=-5Ae-LhMIG0
It is fitting for such a song to use wait on in the pre 18th century sense of await. Shakespeare preferred wait on to wait for, but it was the latter that has become the de facto modern standard.