Funny, I didn't know questions can "expire" here, whatever that means... Anyway, I'm in no rush so if the absence of the "Best Answer" sign doesn't make you too sad (alas, it seems like it doesn't work with these "expired questions" — and, just for the record, I wish I could use it!) then it doesn't matter.
Seriously though, thanks a lot for your answer! Very glad to know that "agus" is never wrong, that's a relief. Well, I think then it makes sense just to pay more attention to the phrases where "agus" ≠ "and"... Thank you, knowing there's probably no clear rule, that's very helpful.