Re: Questions about foam models as safe vivarium decorations
I suggest searching the forum for discussions on question 2. There are many.
As for question 1...if maximizing animal welfare is really your objective function, just go straight to sealing. In other words, proceed from the assumption that you don't want the foam getting waterlogged, because there's a reasonable chance that would result in harm to organisms contacting or ingesting that water.
If other variables like time, cost, skills, etc are equally or more important to you - well, that changes things. Optimization requires tradeoffs, you can't maximize everything. Only a con man would try to convince you otherwise.
The sealant you choose will influence or even dictate when & where you do your sculpting detail - in the foam or in the sealant. You certainly don't need to heat the EPS, if that's when & where you want to do the sculpting. It would be fast, but it's definitely not clean. Back to tradeoffs...
I guess I've skipped the issue of defining "sealing". Something is either sealed to a truly waterproof, no-leaks state, or it isn't. Materials, and their application, both play a role here...but it is definitely possible to seal foam, with perfectly reasonable cost and care. And of course, it's possible to screw it up...ha ha.