Signal protocol prevents replay attacks as every message is encrypted with new key. Either it's next hash ratchet key, or next future secret key with new entropy mixed via next DH shared key.

Private keys, probably not. WhatsApp is E2EE meaning your device generates the private key with OS's CSPRNG. (Like I also said above), exfiltration of signing keys might allow MITM but that's still possible to detect e.g. if you RE the client and spot the code that does it.

Whatsapp didn't implement Signal's protocol verbatim. They appropriated the core cryptographic security and then re-implemented the rest on their own servers. This removes all guarantees of secrecy as long as they can run arbitrary code on the servers they own.

Wouldn't ratchet keys prevent MITM too? In other words if MITM has your keys and decrypts your message, then your keys are out of sync from now on. Or do I misunderstand that?

The ratchets would have different state yes. The MITM would mix in different entropy into the keys' states. It's only detectable if the MITM ever stops. But since the identity key exfiltration only needs to happen once per lifetime of installation (longer if key is backed up), the MITM could just continue forever since it's just a few cycles to run the protocol in the server. You can then choose whether to read the messages or just ignore them.

One interesting way to detect this would be to observe sender's outgoing and recipient's incoming ciphertexts inside the client-to-server TLS that can be MITM'd by users. Since the ratchet state differs, so do the keys, and thus under same plaintext, so do the ciphertexts. That would be really easy way to detect MITM.