Yes, you put both taking lens and Sankor to infinity and then focus with the Rangefinder. Otherwise, you'll be focusing the Sankor to your ballpark length and then finding focus with your taking lens. It gets easier with practice, but you'll eventually want to invest in the Rangefinder.
Yes, you'll need an anamorphic clamp. I recommend the RedStan clamps. They are by far the best.
Yes, to hold the follow focus, which was manipulating the Rangefinder.
The Sankor is one of the cheapest vintage anamorphics you can find. It's the ideal place to start, especially if you already have a GH5.