Soooo hypothetically speaking say if I "killed" Protector Casdin would he be on the same respawn rate as everything else or would he just be dead for good?....

I'm pretty sure Protector Casdin respawns like normal.

I've killed em all before. Casdin never respawns for me. The woman under the bridge will respawn once for me, then never again. 360.

Casdin does not respawn. I've seen him killed by Yao Guai and Enclave. I close the gate whenever I leave to keep him safe and the friggin' idiot storms out to charge a Yao Guai with his Ripper :facepalm: