yes, the lever is in the same room as the airlock, it shouldn't be that hard to find.
Trying to look for it. Could it be that this is some limitation because I am playing a free Itch.io version or something? I mean, I do see the lever "in the air lock door itself". But it isn't a separate object but just part of the door. I rechecked the room and I truly think that, at moment in the game at least, there is nothing else to be interacted with in the room besides: two open doorways, air lock door, and mailbox. The key hook/chain thing isn't an iteractable thing either.
But, thanks for the help anyways :-)