A theory I’ve not seen yet

Cold Harbor is Mark.

Mark is the product. Mark is proof you can take someone, sever them, and they will do your bidding. Even if you parade their ex-wife around, you can make them sit at a screen and do absolutely nothing all day long. They are the perfect slave and the final form of severance.

This is why it's so important. Mark is the final stage in severance.

Irving and Dylan were pretty close; but they weren't challenged like Mark was.

Other parts of the theory:

Helly was there to put the icing on the cake.

Ms Casey is animatronic (that's why she is going to the testing room, why she seems robotic and stiff, why her life is so short). We know that animatronics exist, after all. She's there to realllly test the depths of severance.

The other departments were easing into meaningless work. Mammalians Nurturable proved you can get severed people to do meaningful work. Optics and Design work is meaningless, but at least has some visual value. MRD is pointless.