Yes, if it's an all UA itinerary. You can waitlist for XF. (Well, maybe in your case XC, but that's not clear to me either in this case. Not all domestic flights have C inventory, but some do.) Anyway, you need to call for that.

It's not PA or PB at this point. If you make it to that waitlist after checkin, most people here think you should be on the FFCC waitlist, but YMMV with that. It won't happen automatically, you will have to ask about it, and hope you get somebody that will do it for you.

Sorry, I don't think you get E+ unless you buy it for that leg. LAX-DEN is a relatively short flight for you.

-David