Does Spectrum support mTLS? If so, do we configure the certificate from the client certificates tab in the SSL/TLS menu?

No, as far as I am aware, Spectrum does not support mTLS, unfortunately. This means you cannot rely on the client IP address in the Proxy Protocol header for authentication. An attacker with knowledge of your origin server's IP address could create their own Spectrum application pointing to your origin server and open a connection prefaced with a fake Proxy Protocol header.