How do I seal my shower escutcheon plate to prevent it from leaking?
You should only seal the upper arc (clock face analogy: 7 through 12, to 5, leaving the 6 position open).  

You can seal behind the escutcheon. in the plane of the tile (bridging the actual wall penetration), at the 7-6-5 position.  there should remain a small gap at the bottom of the escutcheon cover plate and the tile.