Yes you can. You should always check for compatibility of products, but generally an epoxy coating can be top-coated.
Link to Sherwinn Williams Macro-poxy with top-coat...
Yes, the poly does not soak into the wood, for the most part, so it can be put on any stain/paint. I just put it over Behr Weatherproofing stain last week.
Yes.
You first need to "screen" it. Just mean a very light sanding to remove the top film and roughen up the service to give the new layer something to bite into. We do this in houses all the time....