Can Enlisted refer to Officers by their first name when off duty?

It's a bit different in the land of Guard and Reserves, Without getting into the legalities (because I'm not a lawyer and I don't want to spend 20 hours drafting a reply), when not in a duty status, we are essentially civilians. As such, absent the military context, yes it would be totally appropriate to refer to me as Vince if I worked for you, if we bumped into each other at (Civilian, non drill-weekend) church, or at the grocery store, etc.

I still address retired CSMs and the like by their rank or with Sir/Ma'am. Even if they told me to address them by their first name....I have difficulty. Even when in a operational environment where we have to use our first names....I still find it difficult.

Personally, I never crossed that line even after I retired. It make a military friendship compromised.