can i redownload it if my computer(s) i install it on gets trashed? because i do a lot of computer component swapping and there have been times things have screwed up really bad
Yes, the registration gets attached to your Stardock account and you can re-download it as many times as you like. It has no activations or hardware checks or any of that kind of nonsense.
can you have multiple cpies of entrenchment per account?
I suppose so, but I'm not sure why you'd want to
You're allowed to have it installed on 2 pcs, you can just log into your account on the other PC and re-download it. Or make an archive and burn it on a dvd, then restore the archive with Impulse on the other pc.