I have a few suggestions about the order of things you listed. In general it looks right but here are some things to think about
1.) Consider removing the lower interior panels as the first item on the list. It is much easier to do if you have some floor to walk on when you are doing it. Also you will find that you need to access the bolts and screws from the top side of the u-channel/c-channel when you are taking out the floor.
2.) On my '73 the only place that there were bolts holding the body to the frame were at the ends of the AS. The only connections along the sides were screws down into the plywood. There wern't any bolts to the outriggers. I was really surprised about this at first but concluded that as long as the plywood is in good shape and fastened securly to the frame and that the channel is screwed to the plywood properly that the body is not really free to go anywhere. The c-channel prevents it from lifting up, the plywood prevents it from pushing inward, and the screws prevent it from moving outward. Also there are a couple of special conditions relative to bolts at the front. I don't know if your '75 is the same as my '73 but it could be. There are two bolts (maybe 1/2" in diameter) that connect from the frame near where the A-frame part passes under the body. I did not see any way to get the bolts out and thought it would be hard to replace them if I cut them off. So I notched the plywood floor around them. The other bolts along the front are welded to the frame on the bottom end. I ended up cutting them off and drilling new holes for new bolts near the old ones. I added lock wasers and nuts on the bottom side rather than welding them in. On my unit these bolts are exposed because of the spare tire well under the front. The back bolts all are inside of the belly pan.
3.) I think you really do need to add something for supporting and securing the body while you take out the floor. There are a couple of reason for this.
A. With all the plywood out and the bolts and screws removed there will be nothing really holding the body in place. It could easily spread out a little and fall off the ends of the outriggers. Even if it doesn't do something as drastic as fall off it might at least move out of position. In my '73 the body on the curbside was about 1" further out than it should be because the floor in that area was rotted and not holding it in place all that well.
B. The c-channel part of the channel at the bottom of the wall is a little flexible with no plywood inside of it and could get slightly crushed making it harder to get the new plywood in place.
I highly recommend the approach that I decided to take for locating and supporting the body while taking the plywood out and putting it back in. It requires a very minimal amount of materials and seems to work just fine. You may have already read about it if you have been looking at the posts but you can find it in the following locations:
Shell Off vs Shell On
several notes especially #74
HELP!!! On a tight schedule, need to replace...
my post #26
Shell-on, frame repair/floor replacement all at once??
There are some photos in my photo area showing the framework.
4.) Some people insist that elevator bolts are the only way to hold the new floor to the frame. Others feel that self-taping screws are fine - maybe closer together than the bolts. On my unit a previous owner evidently replaced some of the floor and used self-taping screws. They were really holding on tight and I had to cut all of them off. It is my opinion that self taping screws will work. I used the Tapcon variety that I found at Home Depot. I also only removed half of my belly pan so for the other half I would have had to use screws rather than bolts. While I think it would be possible to remove and replace all the floor without removing the belly pan I did find it somewhat easier to work on the part where the belly pan was removed. I found it was easier to stand on the ground rather than always having to make sure I had something to stand or kneel on above the floor.
5.) If the floor is too rotted to be able to use it to make a good template for the curved ends you might want to make one out of cardboard or poster board. Even with the front floor out you could make a template from the body as long as you feel it is pretty much in the correct location.
6.) I made a little map of all the holes and notches in my floor and wrote down the dimensions before I took the old floor out. I also took some carefull measurements to decide just how far apart the body should be from side to side before I took it apart. I used a tie-down strap to pull the body into the right location later. All the middle pieces of plywood have straight sides by the way. It is only the curved ends that are not cleanly rectangular.
I don't think you really need to do anything more to the frame than repair any part that seems to have rusted too much. I also think that towing the trailer with the front part of the floor out should be OK. It is my observation that it is the back part of the trailer that is subject to the most flex. Also it sounds like you are able to take it nice and easy anyway.
I would be happy to answer any questions about my experiences with floor replacement on my 1972 31'.
I hope this helps clear up some of the confusion.