Thank you, DrewE!
Would the epoxy do ok with the expanding and contracting of the roof in the heat? I think if it as rather inflexible.
The screws in the roof are small. Little holes.
The screws at the bottom of the roof, holding it to the van, have their heads on the outside. There is a nice fat caulk joint at the bottom, in the gutter. Visible in the pic if I can get the pic to show.
The caulk continues up to the line of screws, covers the heads of many but not all of them. I think there is sealant on the inside of the joint, too.
That is good to know about your cutaway. Yes, I plan to file the sharp edge, and cover it. I am thinking just to use door edge protector strips or similar, rather than upholstery. For now, anyway.