A few thoughts:
I'm not sure a 2x2 wall (I assume you mean a wall framed with 2x2s) will meet code requirements. You should research this.
The peak of the ceiling will be a good place for bass traps (it is another corner), but also you should be aware of where the reflections are being aimed. Read about the mirror trick in the HOW section of this article: RealTraps - Creating a Reflection-Free Zone
Not sure just wrapping the duct in foam will be enough. If you can, frame around it, seal it up airtight, and stuff the space between the the drywall framing and duct with insulation. That will help a lot more.
Yes you will probably want to deaden things, but you will want to do so evenly at all frequencies, which means you will need a lot of bass traps. See here for details: RealTraps - Acoustic Basics