There are several threads talking about the process, though maybe not to the detail of a step-by-step "how-to" tutorial. You might search for "shell off" "shell on" and "floor replacement."
If I were to break it out, it would look like this:
1) Drill out the rivets that attach the bellypan/banana wraps to the shell
2) Ensure that all rivets holding the shell to the C-channels are removed (depending on whether the C channel will come with the shell or remain on the floor.
3) Remove the entire interior furnishings, and the lower interior skins to gain access to the various bolts that hold the C-channel to the floor/frame.
4) Build some gantry lifting frames (see link below)
5) Lift the shell, and pull the trailer frame out from underneath
6) Remove old, rotten floor sections from the frame, but save them for use as a template.
7) Secure the shell, then use the gantry lifting frames to flip the trailer frame
8) Remove the bellypan, saving the sections either for reuse or as templates for replacement. Also remove and discard the old, sagged out axles.
9) Use the gantry frames to suspend, flip, etc., the trailer frame while you are repairing it, knocking the rust off of it, and painting it with POR-15
10) Flip the frame upright again, and install your new plywood subfloor (more discussion on this shortly).
11) Flip again, and install the insulation under the floor. Now would be a good time to install grey tanks too, if that is your plan (and it might as well be).
12) Reinstall the bellypan.
13) Install your new axles.
14) Flip the frame again, put the wheels on.
15) Position the gantry frames over the shell and lift it again. Roll the frame back underneath and drop the shell into position.
16) Rivet the shell to the C-channels, bolt the C-channels to the frame/subfloor
17) Decide what to do with the rodent infestation in the insulation in your lower walls.
Now, for the various questions/concerns you had regarding your plan.
1) Space: I did a shell-off in one lane of my driveway. After landing the shell, I commandeered my open patio area to do the frame work. I have postage stamp sized yard. I realize my trailer is shorter than yours, but you really don't need an acre of land to do a proper shell-off.
2) Working by yourself: About the only task where you absolutely need assistance is with the buck riveting of the shell back together, and the installation of the bolts that go through the c-channel, floor, and frame. So if you can get some help for one day, you can do pretty much everything else on your own.
3) Floor design: The subfloor is a part of the structure of the semi-monocoque design of the Airstream. As such, you want it to be in as big of pieces as possibly (for structural continuity), and you want it CLAMPED between the shell and the frame. The absolute best way to accomplish all of this is to completely remove the shell, install the new subfloor, and put the shell back on. I have read a few threads describing peoples' efforts to replace their floors with the shell on, and my conclusion is that they work twice as hard, and get arguably marginal results for their troubles. Adding a bunch of wooden floor joists to support your floor-board design is counter productive. You will add a whole bunch of unnecessary weight and fill up the bays in your frame that you should be keeping clear for tank and plumbing installation. Plus, loose floor boards, nailed to joist, are just not likely to hold together after a few miles of the whole structure twisting and vibrating. Have a look at the pictures of frames in the link below, and you will see what you are up against. The subfloor bolts to the outriggers, and they are few and far between.
Have a look at the following thread--it shows the design of the lifting gantry frames and what you can do with them. Post 9 and 11 show some good pics of what can be done with the gantry frames. There is also a diagram for building the frames in a pdf at the end.