A set is a set. they film on it for what they need, and when they're done, they tear it down; and if they think it could be used again for some project or episode in the future, of course they'll store it - but no, they're not just going to leave it standing in a soundstage in perpetuity. To a working production crew in general, there's nothing 'special' about it - it's just another set.I don't think so, no. For one I doubt they will want to go to the trouble of building yet another bridge set. The Disco bridge has been modular enough to be used for the Disovery, Shenzou and Sec.31 bridges, but I can't see them tearing down the 1701 bridge. No one is going to convince me they build all that for a two parter.
In any production you build a set for the purpose of getting the scenes you need for the production filmed. beyond that - there's nothing 'special' and depending on your allowed budget, and what level of cinematic flair you want to display, yes, they COULD build an expensive set for one episode, figure they'll never use it again, and junk it. That's the nature of TV and Film production...it's all illusion and nothing is real/permanent.