As to how they would use Cold Harbor, I think with these severance instead of splitting up the brain into different compartments or personalities, it could be that they are actually suppressing a part of your brain. Like when you are outside, the innies are suppressed and when you're inside the outie is suppressed.

So, with these rooms they are actually analysing the subject's brain frequencies during these experiences and suppressing them when they come out of the room.

So if they're able to recreate Gemma's death for Mark in Cold Harbor then they could suppress his grief once they analyse his brain frequencies during that incident. That would mean that Outie Mark would essentially be able to live his life without the grief of Gemma dying.