Essentially, yes you can do this. This is easiest in my opinion if you want to make a first person VR game. You can get a feel for how it would work in VR with a first person controller in the meantime. But, you really can’t accurately program much without an actual headset. There’s just no good way.
Those were just pre-consumer versions. Consumer versions of the Vive work just fine for developing with UE4.
There is no requirement for the game to be on Steam
This is a topic of some debate. Basically anything on the player’s head or hands you don’t need any animations for at all – meshes are positioned and rotated accurately. However, in my opinion you should just avoid trying to do any animations at all, but VR is still early in development. What happens, though, is that having a 3d human model can be very confusing for the player’s brain. Their physical body might be all sorts of places, especially legs. The Vive has no way of tracking legs, so if you attempt to animate the player’s body and it doesn’t match up with what their physical body is doing, it can be quite discomforting.