Others have done a good job of answering your questions, but it were me, one of my biggest worries would be handling the velvet. (Perhaps you've had plenty of practice with it, but as quilters we usually don't.) It can't be ironed like other fabrics, and is very unforgiving about the direction of the nap. Maybe a crushed velvet synthetic would have a lush look without quite as many challenges. I made a floor length, princess seamed prom dress out of that one time without too much trouble. Velvet can be slippery, and perhaps a walking foot would be a good idea. I found a site that has some tips about working with velvet, but it's not focused on draperies per se.
http://www.craftstylish.com/item/554...vet-like-a-pro I think I'd start by having someone hold it up across the room from you to help decide which way the nap looks better for the long expanse. It might not be the same for drapes as for a dress, since people will tend to be looking up at it rather than down. Good luck with this big project! Your booth will be truly elegant.