I've had epoxy (West system) remain good after 10 years + sitting in the garage.
The hardener gets darker, but still works just fine.

I always do the outside first, but don't like making the joint between upper and lower with a separate piece of glass.

One of the forumites - Etiennie Muller- always does the inside first.
I don't understand what he does differently to hold the geometry.
I suspect he builds a few outside cradles while the unglassed hull pieces are still on the tool, then sets those up to keep the shape.

Good luck