I question the need for new messages to help SPV clients find the right path because 1) they seem exploitable to me and 2) all they need is the header chain to find the right path and headers are already short and fast to transmit. Waiting for x blocks again seems to protect them if you assume a majority of well connected miners.
This is a far weaker security model that what is achievable.
If you can point a way to exploit this technique, I'd appreciate having it pointed out:
https://gist.github.com/justusranvier/451616fa4697b5f25f60