Sony's plans are irrelevant to be honest, even as a marketing partner. Deep Silver is the publisher and so the responsibility of promoting Shenmue III rests heaviest on them. Where they go, I expect the game will follow. We already know it's set to appear at multiple events on the run up to launch, and E3 may be one of them regardless of what Sony does or doesn't do in its place.
Actually on the subject of Deep Silver, the implication that the MAGIC showing has been coordinated with them gives me great hope for the content to be revealed. Hopefully saying this won't come back to haunt me, but I feel gameplay of some kind is a lock.
They won't blow their whole wad on an event of MAGIC's size though, so don't set expectations too high for a comprehensive reveal. Some things will be saved for later, or even release.