Nope. I worked for DPL. You can join any Colorado county library. You can also return material at any library, not just the one you borrowed from.
No county has restrictions to using or borrowing books. I've been to several across Denver, Aurora, Jefferson county, Boulder etc. Some county's may require that you hold a different library card e.g. I have a separate card for Boulder, but I dont suggest letting that hold you back from visiting. Just ask.
They don't care what County you live in.
Not as far as I know. I live in Denver and also belong to Aurora public Libraries.
Thanks! Your answer is awaiting moderation.