A * M^-1 (mod G) = a * M * M^-1 (mod G) = a
...
Since the chaincode always comes with the precomputed public keys,
So a wallet file of any type (even watching-only or encrypted) plus
any single privkey from that wallet would compromise
all of that wallet's privkeys... thank you for taking the time to correct me. I'll try to be more careful in the future.
l.j2300,
My sincere apologies for the misinformation, I should know better than to post "answers" if I'm not 99% certain....