Can a properly implemented ed25519 private key with public underlying data be cracked? If the underlying data is public albeit hashed with SHA-512, does that make a difference on the strength of ed25519?  Please quantify the extent.

Can ed25519 be cracked after a certain amount of known signatures?  If so, please quantify the conditions.
No, this does not weaken ed25519 in any way. Known plaintext will not have any effect on a signature algorithm, if it did it would make that algorithm completely useless.