No. This is roughly how TLS ephemeral suites work with mutual auth. Reusing a key pair for signing and DH (or at least being able to) is why TLS KCI attacks wor

No. This is roughly how TLS ephemeral suites work with mutual auth. Reusing a key pair for signing and DH (or at least being able to) is why TLS KCI attacks worked against static DH ciphersuites. Static DH certs basically don't exist, but ECDSA does. The important distinction here is that the long-term keys are signing keys, not DH keys.