Rerouting return air duct in furdown
wood studs that would block the air duct from going directly

I assume you mean straight down in the wall with the mirror on it. I can see the top plate of the wall behind the mirror, and it's doubled. That makes me think it's a load-bearing wall. I could be wrong on that, and if I'm right you'd be taking a big risk in modifying it. So my best advice would be to consult a professional on this one. You don't want to risk compromising the structural integrity of the house.
If you do want to go it alone, to give you a direct answer, and assuming it's load-bearing: The wall is holding up the joists above it, and what's important is that they remain supported. So you'd need to a) cut the top plate in such a way that the part you remove is entirely between two joists; do not under any circumstance leave a joist with nothing under it, and b) place a stud at each end of the opening you create, so that the top plate and the weight it's carrying remains properly supported.
But again, I strongly recommend a pro in this instance.