1. It's fairly stable to move, mine was pulled over relatively steep terrain with no issue. I wouldn't want mine any taller than it is though. If your wagon is a little narrower and you do a couple things different with build, you could probably set it on top and end up with the same roof height as mine.
2. I did not use the lower area for anything. I had considered dumping some rock in before building the blind as a ballast to make the blind more stable. Ultimately I didn't do that because I wanted to be able to move it with an ATV.
3. Movement isn't bad at all with the 4 straps in place. I move slowly when deer are present and had no issues being spotted.
4. My tires were pretty well shot already so I didn't worry about it. If you want to put the effort forth, it will definitely make them last longer to store them inside. Maybe you could stuff those in the hopper?
5. Yes, all the vertical boards for framing are in the pockets. If you look closely in the pictures, you can spot it.
6. Yes, 2 things. The 1'x4' horizontal Plexi glass viewing windows are too long for the way I mounted them. The expansion and contraction is too much and they both cracked. Either I needed to cut them in half and make two windows from each one or mount them in a frame that allowed movement. Also I would have bought a 60" tall blind door from deer view windows instead of the residential door. That would have allowed me to keep the sidewalls about a foot shorter.