Has there been a actual report of an std being transferred by sucking nipples?

No risk at all: In theory, syphilis could be transmitted this way. However, I'll bet there has not been such a case in the US for nearly a hundred years. Otherwise, absolutely no risk at all. They call them sexually transmitted for a reason, meanying intercourse in almost every case. Don't worry about it.

No: it is perfectly safe if you have permission.