First off....here's
the RMAF site.. 
What time on Friday will the rooms be open for listening? and what times will they start to pack up on Sunday? I'd like to squeeze in as much as possible in 3 days.
Friday, October 10: 12 PM - 6 PM
Saturday, October 11: 9 AM - 6 PM
Sunday, October 12: 9 AM - 4 PM
I'm hoping to fly out Thursday and leave late on Sunday.
Good idea....

How close is the Denver airport to both hotels?
About 30 min.'s time....
Is a rental car even needed?
No....take the Super Shuttle van....right outside of the terminal doors at the airport....quick and easy.
Do I need to preregister or buy admission tickets in advance?
You don't need to...but its better to...then you get your pass faster.

The show is even bigger this year Rob....have fun....see you there.

Chris