Oleg, I used this adhesive
https://www.homedepot.com/p/Liquid-Nail ... /100171523About a gallon and half. Let's just say I'm glad it's on the bottom, because it didn't go as smoothly as I thought it would, though I think it'll do the job for the underside.
Should have realized the plastic lock nuts wouldn't work for welding. Thanks again for the idea.
As for the other queries - Thanks for your suggestions and troubleshooting. I'm planning something like the lift system shown in the vintage plans linked in the first post (I think) of this thread (close to the end of the article/plans), but hung from the ceiling. My garage ceiling height is too low for that to serve as permanent storage, though - I wish!
The side storage is a bit crazy, I agree, and I'm hoping to not have to take it on/off too much, but I think it's a must for my scenario, at least at times.
I have already had to put the bottom assembly up on its side and push it around by myself on dollies a number of times. That component is definitely the heaviest part of the cabin, though there is still plenty to build, so it's not a true representation of how tough it'd be. I plan to have it strapped to the wall it's stored against when it is on the side.
Nonetheless, I hear you all and I am leaning more towards leaving it on the trailer and parking it on the street some of the time (all of the time?). This is more tempting since I'll need to install a bolt-on tongue extension that, when on, won't allow the trailer to fold up, so that complicates the removal process a bit more. There is no doubt that it will be a hassle to get on and off.
Still unsure about the street, though. I live in a big city, so parking is pretty tight and theft is a potential issue. Nonetheless I have been noticing a few more little trailers in my neighborhood, which is reassuring. No side yard or such and storage is too expensive.
Here is a view of the top side:
After this photo was taken I put ghcoe's thickened primer mix on yesterday and it worked well, though I need to give it a light sand. Thanks again for all the feedback.