Is it possible to discover the private key for a public address given one possesses the public key and the encryption passphrase?
No, it is not. Public keys are derived from the private key. Imagine if I had your public key (which is public information) and all I needed was your password to gain access to all of your funds. That would be extremely insecure. A simple brute-force password guessing system could handle that in a couple weeks, depending on how complex your password is.
The only way to reconstruct the secret/private key is using the seed-words (mnemonic) + passphrase (not same as encryption password).
As @utdrmac mentioned, it is not possible to reverse engineer the private key from the public key. Public Key is created from a Private Key using one-way functions (also called trapdoor) which is easy to do but the reverse process of finding a Private Key from a Public Key is practically impossible. Sharing a few links below to give you a better idea about how Public/Private keys work in asymmetric encryption:
Hope this helps.