So I learned some new information about "The Bay Monk," or as I guess I should now be calling it, "Bay's Monk." See the name comes from Bay Raitt, a 3D modeler and animator who worked at Valve from 2014 up to 2013, and worked on HL2, TF2, the Episodes, the Portals, and the Left 4 Deads. This guy is actually one the artists responsible for Gollum's face in Lord of the Rings, and you can really see the resemblance when looking at his Father Grigori sculpt. As for why the model itself exists, it was probably done as a test so that he could gauge his skillset and abilities when working in the source engine after he was hired by Valve. I wouldn't be surprised if the model itself was never actually considered for use in the game, but was instead just left in the files of the asset repo by mistake. So there we have it! The whole story of this peculiar model. A special thank you to Gmadador for giving me the information about Bay Raitt, I had no idea until she pointed it out.
I can't be the only one who thinks Grigori's voice suits this design more.
Maybe whenever the player isn’t looking, Gregori takes off his mask to reveal this as his true face.