Hello 

1. No, system allows a user to pay again. I think the user should know what membership level he currently has.

2. Payments for membership levels will go to an admin/operator profile which is selected as 'Main seller' in Studio -> Payments -> Settings -> 'Main seller (To sell on behalf of the site)' setting.

3. [+Add Product] adds new product in Market app while Premium membership level is controlled with Paid Levels app. They are 2 different apps which work (add, edit, sell, etc) with different content (products and membership levels accordingly).