I guess I missed the part about the tapered threads. They don't seem to show in the drawing. The only way I know to do that is with a taper attachment, so that may be your next project. Sometimes you have to build the tool to build the part.
Since that has to be built in one piece, then the only way to get the 2* dihedral I can think of is to extend the arm center lines out to the opposite end and put the center at the intersection. That may mean a little extra material at the end that you will cut off in the final piece. I think you would have to do this in a CNC lathe also, unless you can find someone with a multi-axis horizontal machining center with live tooling. An angle plate, with a 2* taper built in and the center block bolted to it, attached to a face plate might do it, but it would have to be built pretty stout.
I'm pretty sure this whole job can be done on a manual lathe, but some practice pieces would be in order. Maybe a chunk of free machining plastic to start, like Delran or something, just to refine the technique. I assume the final part will be 4350 or similar alloy.