Its a GRAS food additive iirc.
As far as iso/etoh, its just the thinner for the shellac.
Not too sure about the time delay, as elemi was just a bunch of BS imho. The idea behind it was that by inhibiting certain enzymes using other essential oil's you could get mescaline like affects from elemi, because you know its looks so close to mescaline structurally lol. IMHO it was a scam, since the dude that pushed the theory HARD, had a shop where he sold all these oils, along with extracts of harmine/thh of questionable purity/legitimacy. I never got any affects, and it was the only time i tried it for time release with drugs. Wiki has some info on shellac about enteric coating though, so that seems legit.
How i did it, was fill 00's with elemi oil, dip the bottom half in etoh/iso thinned Bulls eye brand shellac, let it dry, put the top cap on, and dip that right below where the bottom layer of shellac was to ensure a good seal. Thats all i really know about it as an enteric coating, no idea about time/double dipping. It was just something i tried once. I do remember not getting those nasty elemi oil burps the time i enterically coated the gelcaps though, so i assume it worked.