Rog,
You're making great progress!
Regarding your floor, my teardrop is 58" wide. With 2" x 2" frame rails, that leaves a span of 54" for the floor. I only have two crossmembers on the trailer. My floor is 1/4"-3/4"-1/4" insulated sandwich. The 1x2 floor framing is about 16" on center.
You may want to consider is the top thickness of plywood. When I step on the floor, there is no deflection for the 54" span, but I do feel a slight sag when I step on the 1/4" plywood between the 16" spacing. (I do weigh around 200#.) Of course, once the teardrop is finished I won't be stepping on the floor. You might keep the 1/8" flooring, but reduce the floor framing in size to 3/4" x 3/4" and space it at 12" on center to avoid this situation.
For your frame rails, I think your idea will work. The farther along I get in my build, the more convinced I am that most of us way overbuild our teardrops. The "box" is incredibly strong once all the walls, bulkheads, floor, and roof structure is fastened together. The trailer is just there to give the axle something to fasten to and to tie the axle to the tongue.
Tom