[Accelo]

Yes, epoxy will stick to the smooth metal. I would use 180 to 220 grit. 80 may be over kill IMHO.
The blaster may consider re-blasting it but that bridge may have been burnt.
I am not convinced it's necessary.
It would be a very different situation if it was left in the rain for weeks.
Multiple solutions and their isn't one single best cure all.
Pick the one that works for you. I do recommend using a self etching primer as a prophylactic step in-case you miss a little rust.
The fact the blaster left the some of the seam sealer I take as a good sign. Blasting it all out damages the surrounding metal more than necessary.
Looks like a great start on a project. I am retired and mine took 28 months to running driving.
I named my truck PITA. I can guarantee their is more frustration to come. Thou shall overcome.

Cheers