I can think of a couple of ways to implement what (I think) you want to do—be able to play either the Reface or the MBP from the Launchkey, and be able to switch between the two (on the fly, like for a live performance scenario?)..
First off, you're right—the Quadra Thru would let you play both sound sources (at the same time) from the Launchkey, but to be able to play one *or* the other you'd need to either mute the Reface and the MBP alternatively on their front panels, or run both their outputs through a small mixer (ideally with a solo function), for faster switching.
Alternatively, I would think that if you set up a track in Ableton to use the Reface as an external MIDI device (via the track’s In/Out section or the External Instrument device), then you could treat it just like the other VSTs in your Live session, choosing Reface simply by changing tracks in Live. Unless there's a reason why you'd need to directly trigger Reface, I think this would be the best (and easiest) solution.
Btw, since this is unrelated to MainStage, I've moved this thread to the Ableton Live subforum.