I recently bought a 3d printer (3-4 months ago). So I have some experience with this. Here's my take on it.

1. Sticky surface, IMHO opinion he didn't clean/cure it properly. I'd try the 90% alcohol. I use 95% denatured alcohol on all my printed parts. I get it at Lowes
2. The rough surface is best handled by first a coat of primer and the sand with about a 400 grit. It will cover most of it. It you might have to put on a skim coat of filler and reprime sand again and you should be good. Your cousin's suggestion is also an excellent approach.
3. Any super glue should work. I buy all mine from the Lowes down the street.
4. Just about any paint should work just fine. I've never had any issues.
5. If he'll tell you ask him what printer/resin he's using as it could give further info on how to clean it etc.

I'd love to see some pictures if possible I'd love to see some pics of the raw resin.