Welcome to the Forums!
I'll address your questions in order of appearance:
Q: im seeking sound advice on where to go next with the frame (reinforcing, rear seperation prevention, rust resistance, axle/shock assesment)
A: Well, you can either just repair your frame so that it is like it was when manufactured. Those rotting outriggers can be purchased or made from scratch. The Argosy frames were intentionally made lighter, more spartan than the Airstream frames, so you could "upgrade" your frame to look more like an AS frame of that era, by adding outriggers and putting a cross member every 2' and eliminating the wooden stringers that run from front to back. There are lots of pictures of AS frames on these forums to give you some guidance. There are several threads detailing the best practice for preventing rear end separation, which amounts to putting an L shaped flashing inside the rear end skin and extending the base of the L out under the bumper trunk. Rust resistance can be achived by painting the frame with POR-15. If you have the original axles and shocks then they are definitely shot. The torsion axles only have a lifespan of about 25 years. If you aren't sure, then one way to check the axles is to jack it up and measure the amount of travel the arm makes as the weight is removed from it. 12-3 inches of travel is good, less than that is bad. Lots of threads detailing how to evaluate an axle.
Q: Wisdom on shell on/off controversy.
A: I'm not sure there is controversy. Many folks use a gantry or some type of jacking to lift the shell, some folks replace the subfloor with the shell on. I suppose it boils down to personal preference. Some people think they don't have the space to take the shell off, some are just intimidated by the process, and feel more secure with the shell sitting on the frame. My opinion is that doing the floor and frame repairs with the shell on is doing it the hard way. There are lots of things you can do with the gantries if you go that route. Again, lots of threads showing frames suspended from gantries for painting or repair, etc..
Q: Whats my best options on how to go about repairing the c channels/outriggers.. where do i buy them?
A: I have bought both C-channel and outriggers from outofdoorsmart.com, but I would expect other AS dealers sell them as well. There is no rocket science involved with the outriggers, any competent welder ought to be able to fab up something that would do the job. You can also buy factory cross members from the above source, if you plan to add crossmembers to your frame (or, again, just have the welder fab them up from some angle iron).