From my experience with facial rig and UE4 setup:
The best way to easily share the facial animations between characters ( all of them using a Blend Shape rig ) is to build an additional, standalone rig, which is driven by the blend shape values, here is an example ( using a lipsync setup ):
By doing this you’ll be able to drive the blend shapes in realtime onto a character and share the standalone rig with few steps, rather then have the animations tied to the characters.
In addition to that I also developed a setup in Maya which allows for easily rig sharing between different characters
I’m currently working on a tech demo for my VR project, which will feature a full body mocap suit, combined with a VR headset, and the player will interact with another human, who’s also wearing a mocap suit and a facial tracking camera to read facial movement using Faceware Live for UE4, so I really can’t wait to test everything all together 
Since you’re also talking about alternative solutions for the facial rigs I strongly suggest you to try Fabric Engine, which is integrating a lot of useful stuff for game engines