Yes, I use CS as a music download store and I have several seperate downloads per product.
You can set permissions for admin users, I have not used it so cannot comment but you should be able to try it out on their demo store.
Not by default but I’m sure the CS team could integrate for you at an extra cost. I have used their customization service in the past and it is very good and well priced. I have recently been discussing forum integration here and I think I’m going to keep the store and forum seperate, at least for the time being, in the past I have integrated 2 different systems and it gets complicated when it comes to modifying users in either system and/or upgrading the systems themselves. But if it’s what you want it can be done but is not a default feature.
Hope that helps in your decision. CS cart is probably the best avaliable, especially in it’s price range.