I believe I covered the re-org depth mitigation in the OP? It doesn't help for two reasons:

1. The re-org from this attack could easily be less than the maximum depth
2. Re-orgs greater than this depth will still be accepted by all syncing nodes, and the the attacker can impersonate a majority of nodes for ~0 cost, leading to all syncing nodes accepting his version of history, which eventually leads to it becoming the canonical chain.