I have had a number of race car trailers with similar construction on the roof to a TC. My favorite had 3/4" plywood under the aluminum which made for a nice walk on roof, but a bit heavy of course. I would suspect you could easily get away with 1/2" as well. Current trailer just has .030 aluminum with nothing underneath. If you work on top you have to step carefully from rafter to rafter. I took a can of spray paint up there and marked the rafter locations so I could walk around easier. Also, having wood underneath makes everything a whole lot more solid when adding things to the roof like tank vents, refrigerator vent, tv antenna, etc.. In all cases the aluminum is one piece, it comes a standard width in a roll and you buy it by the foot. In a perfect world the top aluminum is folded over the edge, then the trim covers that and the screws run in from side so there are no holes in the top, and then a bead of sealer between the aluminum and the trim. I just spent a whole day resealing the roof on my Pace trailer last weekend (10 years old, it was overdue) and I was surprised to find that the roof metal on that one did not overlap the edge, it just lays flat, and had the trim screwed down from the top. They just used a wide band of sealer to cover the joint and screw heads. Would not be my first choice, but it seemed to work well enough until the sealer got older and started to crack. I have never had the roof glued down on any of my trailers, I don't know if that is a good idea or not. I know my roof really expands a lot on a hot day where it is noticeably warped looking as opposed to nice and flat on a cool day. Perhaps the metal needs to be able to move around a bit for expansion. Just thinking out loud there. I do think it would be quieter while moving in a TC, aluminum rattling around is surprisingly loud when you are driving, I always use some silicone behind a piece of aluminum before I screw it down to keep it quiet.
I would not even consider building one without insulation in the roof. The more the merrier. I've had trailers with and without and the ones without insulation are ovens in the summertime even with the doors open. The ones that were insulated were always nice and cool inside. This Pace has wood strips on the bottom of the roof square tubing to space the ceiling down an extra inch leaving about 3" for fiberglass insulation. But I don't really think you can have too much.
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