I've been dealing with the upper arm issue by wearing a short-sleeved blouse over a tank top. Linen works nicely if you can live with a few wrinkles, and I think there's some rayon that drapes nicely and sheds wrinkles. I get ventilation without exposure, and it's easy to button up if you get chilled. Have two pretty new J.Jill pin-tucked linen blouses that have gotten compliments worn this way. Sounds like there's a J.Jill fan club on this board!

Los Vegas may be hotter than Orlando but it's way less humid so probably won't seem as hot as it is. Though you'll be in A/C most of the time, so a light cardigan and even a pashmina (don't leave home without it!) are a good idea.