Here is another approach that fails for a different reason.  Perhaps the two failures can be merged into success, but I have not thought about this too deeply.

It suffices to prove the claim for shrinking just one blossom.  Now, we instead view the alternating path $P$ as starting from $v$ and let $u$ be the first vertex that it meets the flower with the blossom.  Note that the edge right before it meets this flower must be a non-matching edge.  Now if $u$ is in the blossom, then by proceeding backwards to the root of the flower, we get a path from $v$ to $X$ in the contracted graph.  This is also true if $u$ is not in the blossom, *unless* $u$ happens to be adjacent to the root.  In that case we cannot proceed backwards to the root. End fail.