I'd say yeah, probably epoxy. Superglue is a possibility, I guess, but it cures dang fast, so you don't have much time to straighten/position that grip once it makes contact with the tube. Of course, if things are laid out and marked beforehand, the quick curing could also be a big help.
About the rivets, I also don't really care what they were for, but I think to reinforce the D ring/bottom plate is a pretty safe bet. I don't think an old photographer would have much trouble finding a replacement spring for the batteries, so I feel like that theory has less logic behind it. And I don't believe that four rivets all on one side of the tube, would do much to keep the spring retained in the bottom of the flash tube. I feel like if someone were to install rivets to keep the spring (or batteries) in place, they'd be placed say, more like a cross. Or a triangle, if you want to use minimal "rivetage".
Danny, I don't think the productions were quite so rushy-rushy/slapped-together and out the door as you believe. Perhaps with some things, or in some circumstances, like while on-set/shooting, taping the Stormtrooper armor back together, but for the most part, a good deal of thought and care went into these films. I'd say the prop fabricators definitely gave a damn, and did everything they could to do the best job they could with the time they had. Especially in pre-production.
I will say I'm excited as all hell to see this pic of the Hero D ring that y'all are passing around!