Do ShareTopo in SC as it's more reliable. It's also one less step in Fluent Meshing for whenever you do a mesh update.
Correct, assuming the faces connect then you'll have an interior face as opposed to two interface surfaces.
We can run mrf in steady or transient: sliding is transient only. The reason for using one of the other is whether the rotation changes something physical in the flow. Ie if it's a mixing tank impellor and there's a close pass between the blade and baffles you need sliding mesh. Otherwise the averaging from mrf is fine, and it's computationally cheaper.