In terms of the Atelier and the Mansion, the menus are totally distinct, although there are certainly similarities in style. At the Atelier, the bulk of the seating is counter, and the menu is basically a series of small plates to pick from, although there is also a prix fixe option, I believe. The more formal restaurant is quite formal inside and the menu is a prix fixe of 9 or 16 courses, as I recall (although you can also a la carte). There is all the usual fine dining stuff, including a great bread cart and lots of little extras. It's just a different experience from the Atelier.
Rosemary's is still there; another good local option is Todd's, which is on Sunset in Henderson. In terms of Picasso, we ate there recently and were not terribly happy; I have had better (albeit less formal) meals at Bouchon and better high end food at Alec in the Wynn