Okay but Cold Harbour is the episode that really, truly, fully humanized the innies - all of them, but none more so than Mark S. Let me digress.
I was so goddamn excited when iMark and oMark started talking with the camcorder, and it was so clear that iMark was incredibly touched at the start, that his outie had been thinking about him, wanted to talk to him, apologized to him. But oMark fucked up BAD the moment he belittled iMark's relationship with Helly and got her name wrong. It mirrored Helena getting Gemma's name wrong in the restaurant, and it was the same flavour of disrespect and disregard. oMark obviously isn't anywhere close to as malicious as Helena, but this scene made it extra clear that he doesn't see iMark as his full equal, and iMark realized it too.
So when the time came, it made perfect sense that he acted how he did. Mark nearly died, accidentally committed murder, and more in the span of a few minutes. iMark rescued Gemma, saw her safely out the fire escape door, and in doing so discharged his duty to oMark. He was in an insane and impossible situation, somehow managed to accomplish his mission, and then once that was done, he stepped fully into his agency for the first time in his short life and made the choice HE wanted to make!!!
I completely and fully disagree with people saying that that choice didn't make any sense. Of course he and Helly don't know where they're going. Of course they don't know what they're doing. But they were ready for death, had had their final parting, and then iMark said no!
I felt horrible for Gemma, and I cannot begin to imagine how she would have felt seeing that - it absolutely broke my heart. But at the same time, I couldn't help but feel proud of iMark, in a way, because in that moment he stepped out of oMark's shadow and became his own person.