Yes you can. You can rent boards right outside of the park (on the right, before the park sign) at Great Sand Dunes Oasis (privately owned).
Yes, i saw several people doing it.
Yes! The outpost just outside the park rents sand boards (stand up) and sand sleds (sitting down) for $25 a day. Must be retuned by 6pm. Several people try bringing plastic snow sleds and snow disks, but they don't work. The sand needs a hard smooth surface, no groves like snow sleds.
Yep! Rent some at Sand Dunes Oasis! Don't know if there are any others, but I do know you have at least one option!
Yes you can rent from visitor center
Thanks! Your answer is awaiting moderation.