I wouldn't shoot anything with variable ND if you want your people to look like people. The polarizer isn't worth the convenience.
Depends. 50mm is/was pretty popular on movie cameras too, but I think the 35mm is more versatile for a smaller sensor. The cheapest 35mm lenses tend to cost considerably more than the cheapest 50mm lenses though.
You can build a decent wireless follow focus with a $100 three-channel RC car radio and about $50 worth of parts, and you won't have Lego on your camera.
I don't believe a teleconverter with no loss in brightness is possible, no matter how hard Fuji is "working on it".
The A7RII is arguably a more versatile video camera than the A7SII, especially when you add a speed booster. I wouldn't count on it.
You mean step down?