Laurie,
What a neat gift! Here are a few of my thoughts:
Yes, you can sand the top just like you do the sides. As you know though, resin can sand down fast, so start with a fine grit sandpaper.
How big are the imperfections? Maybe you can dot in a bit of resin to fill in the dents, then go back over them with a bit of sandpaper to even them up. (I wouldn’t start with anything over 600 grit — go slowly — easier to take it off than put it back on.)
Of course, I would ask whether or not this is something your husband would notice. Im super fussy perfect about my resin jewelry too, but I have found out that I drive myself crazy over things people don’t even notice.
As for the shiny gloss finish, you can get that by either propping up your piece and coating it with another layer of resin or you can use the resin gloss sealer spray (find here: https://shop.resinobsession.com/collections/tools-and-supplies/products/castin-craft-gloss-resin-sealer-spray ) You can see both of these techniques in the video on our you tube channel on how to sand and finish a resin bangle bracelet
https://youtu.be/51kSn5Xqfw8