Your car is 3 years newer than mine, so my experience will not be much use. However, I would address some of your queries to your M-B dealership - I've found them very helpful.
Re 1. I doubt if DVD copies will work. My Satnav is CD based, and an updated CD is around £100.
Re 2. It's not quite comparable, but my COMAND has a TV feature - but the TV module is not fitted. I have read a DIY post here about unlocking COMAND. A search should find it, but it does not work on my version.
Re 3. I wouldn't go the eBay route & I have no idea how you would program it.
I also needed a spare key 2 years ago. The dealer failed to get it from the original owner, and it occurred to me that using this key my car could be nicked.... But if you go to M-B they can cancel that key's code and issue you with another key suitably programmed to your car. Costs about £100.