Generally not, because this will make the cost high if international delivery is used. As a common practice, you will get your permit after your arrival in Nepal, and you enter Tibet by train or air. Your travel agent will deliver the permit to the hotel that you book in a specific city; you must provide your agent with the name, address and phone number of the hotel.
No, you cannot have 100% independent tour in Tibet; instead, you will have either a private tour or a group tour including a guide, driver and etc. Even before 2008, independent tour in Tibet is rare, and after 2008, the restriction has been reinforced by adding many checking-points on the way, particularly if you travel outside of Lhasa or visit some sensitive areas. There are always someone checking your permit, and you must have a tour guide to help you go through those formalities.
Generally speaking, from April to October is the best season for Tibet tour. However, having a winter tour can be pleasant if you don't mind the cold weather. You can avoid the tourist crowds, and economical accommodations and transportation are more available.