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
I’m unable to get any info on what my grandpa did after leaving active duty and going to work for LM on government contracts. I have paperwork mentioning him, and it’s alllllllll still sharpied out almost 70 years later. Dude was a logistics engineer, he basically organized warehouses, yet apparently was so important to the nuclear sub program (Mare Island in the 50s & 60s tells me that much) apparently that I’m not allowed any further info
It’s entirely possible he didn’t know what he was working on, I only have guesses because of other shit we know from decades after his death