Is it possible to implement this on all nations like it's done here for the questions and suggestions? Is it accomplished via style sheet trickery, or is there some way to designate which users' responses are flagged as "official"?
Yes, you found where to do that, it's "edit the suggestion" - We intend to make that a bit easier, but that's the way it works for now.
There is no "pending." There is "planned", "not planned", "considering", "started", and "completed." You don't have to use all of them, if something is never completed, it's never completed.
I could see another situation where you might want different language to refer to a legislative process, like "in committee" or something along those lines, but can you describe how you would use it? I tend to not want to let people change things like this because it just adds to complexity and if there is some way to figure out how to make it work the same for everyone it ends up being better for everyone.