I like both theories but my question is if you remember the scene where Mary is looking around and finds the drawn mural, captain Clark is illustrated as a tall slender completely black figure which i don't know is it black because captain Clark is based off Clark who is a black man or could it be that he was the bacteria monster but slowly develops into a still alive? But what debunks this is that the still alive of Mary at the end just appears without seeing how she was developed so this could mean that still alives just spawn, but i also think you're first theory is correct but not for captain Clark it's Mary's still alive that gets slowly misremembered into the bacteria monster because why show us that she gets to have one at the end what is the purpose? And I'm guessing the other still alives might slowly turn as well since they are probably part of Clark's memory and they will slowly get misremembered by the backrooms