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/451616fa4697b5f25f60I'm a bit shocked that no one is talking about this idea.
It would greatly increase the security (and hence the value) of SPV clients and allow far greater scaling with almost no sacrifice in security.