Need help framing a wall in basement

I need to put a wall in my basement for a bathroom but have many obstacles to overcome. It has to go in this spot because I already have the shower pan in (curb less) and the plumbing is behind that wall too.
Image 1 is where it is going, image 2 and 3 show the obstacles and if you look closely you can see the red lines of my laser level where the wall goes with framing on the backside of the line.

The wall is parallel with the ceiling joists but offset so was planning on putting 2x4 nailers across them every 24" but can't in about half of the area to the left because of the ducting. I need to build a soffit around the vent pipes anyway because they will not go up closer to the ceiling but not sure how to attach it all above which is why I'm here!

Not sure how to describe but should I build the framing around the pipes attaching the inner part where the soffit would be and then over to the wall framing and that would suffice? I have to build soffits elsewhere and was going to use a method with 1/2" osb and 2x2's, a 6" drop but those are simple comparatively.

Please let me know if I am not explaining correctly and thanks for your time.