When you say "this would be a massive problem if there were any way to use the public key as an input to crack the private key (or use it to verify)..." do you mean that there is a way to use the public key to crack the private key? Does this relate to the advice I have read but not quite understood the reason for, that paper wallets should never be re-used? (putting to one side the "change" issue which is a separate point)
There is no way to find private key when you know public key (it means that ECDSA is not broken). Bitcoin private key is protected by ECDSA, and public key is protected by SHA-256/RIPEMD-160. When you do a transaction, you publish your address' public key, so the address becomes protected only by ECDSA, what is still perfectly safe.
