When I've used Mopane wood before, it usually sinks either right away, or soon after soaking. -It's some pretty dense stuff.
As for epoxy, I imagine most types should be okay, but I'd avoid the ones that are 'metal filled' (J*B weld), or anything that claims mildew/mold/bacteria/microbe resistance or whatever. I think most of the standard 5-20 minute ones should work fine. I could be wrong on this, but generally epoxy(once cured) is pretty inert. It's what they use to coat gravel to make it all the various clown-colors, to fill/waterproof submersible motors and electronics, and people also use it to coat the inside of DIY plywood tanks. I've also used various types to make backgrounds and paludariums and such.