Will polyurethane or Osmo over cured epoxy add shine or scratch resistance?

No. UltraClear Bar & Table Top Epoxy is a commercial-grade product designed as a standalone finishing top coat. Polyurethane is not more durable or robust than the epoxy itself, so applying it will neither help nor hurt the epoxy's scratch resistance. There's no reason to polish the cured epoxy and coat it with poly, and since the epoxy cures to a high-gloss, glass-like finish on its own, a gloss product over it is unnecessary too.

Osmo Polyx-Oil is the same story on an all-epoxy surface: it won't add any shine or protection. Osmo is meant for pieces with exposed wood, such as a charcuterie board with a river of epoxy or a river table that won't see much use, where you don't want an epoxy finish over the wood. It bonds to the wood and acts like a standard wood finish such as varnish or polyurethane, and it doesn't offer the same level of shine or protection as the epoxy.

If you want to coat a spot you had to sand, oil-based polyurethane is fine over cured epoxy. If you'd rather restore the epoxy finish itself, re-pour the flood coat: scuff-sand the epoxy with 220–320 grit, clean off the dust with acetone or denatured alcohol, and pour a new flood coat of Bar & Table Top Epoxy. It self-levels and fills in the sanding scratches.