Recently I’ve played dead space and some other survival horror games like Alien Isolation, and just about every single time you see someone in there, they’re somehow invincible. Like, the fuck, that fat idiot inside that VIP Area there has somehow managed to lock himself in that room for what, weeks? Months? And somehow never did any of the necromorphs even notice him? And that crazy scientist guy? He can just run around in the ship? And what about alien? The medic dude, like, there’s a vent right in the room next to him. And he has a broken leg, so the alien can even smell his blood. Also, what about the marshal? Well, thats kinda fair game, he has an effective weapon after all.
Dr Mercer was advantageous to the Marker to have alive and doing his work, so he isn’t killed until his usefulness has run its course
Canonically: the markers try to influence people to do what will assist in Unification. Isaac vehemently doesn’t believe in Unitology so the marker throws enemies at him to try to stop him / manipulate him into doing what it wants (in DS2 & 3). Other people in the series are believers, so they don’t need to be killed off if they’re more useful alive for the time being
Survivors lasting by staying behind locked doors isn’t too unbelievable, either, given the fall of the Ishimura took a while (1 month from marker discovery to it getting on board the ship), and you arrive not that long after it falls (less than a week). It’s been at most a week since she went dark, I think, which is a believable timeframe to survive given the comms going black is at least a day before the big fall of the ship
Oh hey my memory was pretty close: Check the year 2508 section
Other games absolutely pull that shit too much, but DS did a pretty good job with background story details to explain why some people aren’t dead
Was going to say this myself, but you already did it better. Mercer didn’t survive the enemies, he was one of them.