Hey, question's are great. It's helping me get thoughts out of my head too, so they're super helpful.
Can ghosts swim? Or is water also made out of the same immutable material?
No, they're can't swim. It's not possible for the water to part around them, after all. However if the water's perfectly still, they can walk across it. Smooth waves would push them up and down, but storms and water crashing down over their heads is a death sentence, so walking across oceans is not a good idea, but lakes are great. Heck, lake-based ice skating might be a thing depending on how friction works in this universe. Gravity also continues to work, so friction likely exists too.
Would they wake up from death underwater (or in a collapsed mine) to be stuck in the material, continually dying forever?
Yes. It's actually a bit worse for the collapsed mine. Since the ghost would probably end up in the same position as their body, they'd probably just be stuck there, screaming, until the corpse eventually rots sufficiently for the rocks to come down and crush the ghost. And
then they'll be dying forever.
Could gases/fire hurt a ghost?
Good question. I'm not sure. Fire hurts because of burning and/or heat, but I'm unsure if ghosts are affected by temperature and they're also not going to be fire's fuel. On the other hand, fire still occupies space... it might be that most ghosts still keep away from fire because fire = bad when they were being educated. I'll say for now that fire doesn't hurt ghosts since it'll just be the same as them attempting to poke a sculpture. A sculpture that moves very quickly, but a sculpture nonetheless. If I decide that temperature exists, then I'll come back to this point. I don't think it would though, at least, not in a manner that's tied to the living layer.
Are ghosts physically the same as when they died? So there are paraplegic ghosts in wheelchairs, baby ghosts who can't think/speak, and old ghosts who have trouble getting up after falling down?
Okay, so this is interesting. Yes. That's my answer to the question, but when I start following through the implications, there are some which I find interesting and want to exist because
themes, and others that are fiddling and I'm not smart enough to totally make sense.
One thought that I had was about hospital deaths. Logically, if ghosts can see into the living layer at all times, some ghosts are kind enough to watch out and assist those who they see are about to die (and by assist, obviously they can't prevent deaths, but they can talk to the ghost when they wake up, calm them down, direct them to family, and so on). So deaths in childbirth that happen in hospitals, there's likely a network of ghosts who work to look after parents and children. Ghosts in the same layer are able to touch one another, after all.
I was on the fence about if I'd like aging to exist in the ghost world. For the case of children with developing brains, for example, I'd like for it to happen. But if death by old age exists, then we hit the fridge horror of a ghost growing old and then apparently vanishing. That's a neat idea, and I'd like the quiet wondering of "hey, why does the protagonist never see old ghosts," but I don't know if I want to commit to it. Because, the idea of ghosts being frozen at a particular age also has appeal, as does ghosts who only ever knew being ghosts. However if aging is introduced, it implies there are bodily processes going on too, which... no, that makes no sense, and I'm not dealing with "what do ghosts eat" in this.
So I'd say that yes, there are babies who are stuck. Maybe after the considerable human history ghosts have worked out a way to help things out, but until I can think of it I'll probably not have the protagonist ever get an answer. (Plus, it allows a scene of the protagonist wondering about this and then promptly being very uncomfortable by the idea).
(
but wait if there's no bodily processes then no ghosts would be able to create memories-- gah. I need to draw a line
somewhere).
With regards to physical limitations, I hadn't thought about it but I'd say yes. Depending on the I'd like to note that ghosts awaken only with the clothes on their backs, so, there's no wheelchairs. Strictly speaking ghosts should end up waking naked, but I don't want to deal with it, even though it raises the questions of "so how do they actually get those clothes" and "what happens if you die naked."
Hm.
I was going to bring this up later, but one vague concept I have in mind is that if ghosts travel upwards a lot of layers, say in either the hundreds or thousands (or millions) depending on how plausible it would be, the connection between layers and reality starts to break down. I know, for example, that as you go upwards, light sources in reality grow increasingly dim, sounds are increasingly muted, and/or it's generally creepy. It might be that in some layers, physical, touchable, non-living matter can be acquired. Like... if you're in a clothing shop you can go upwards and upwards and eventually, in pitch blackness, the shirt you're touching
moves under your touch. Good luck getting back down though. There might be rumours, unfounded or not, that going upwards significantly might stop you going back down again, and there's always the real world still there and something could kill the ghost and lock them up there in the dark. I dunno. It's a nice worldbuilding feature, but I'm unsure if it would ever really come up in the story or how feasible it is. (Then again, if the ghostly supporting character gets stuck somewhere, they might end up hearing rumours about layers breaking down if you go high enough, and end up in a scene where they go up, and up, and up... and eventually push straight forward through the door. And then scramble back down again. Or something. Also, it reduces the fridge horror of the ghosts who die in places where they'll re-die, since eventually they'll be able to get out. They'll just end up in darkness and will never know why they've been damned to a terrible fate, unless they run into someone who is very, very old). It's nice to iron things out, though. This ended up far longer than I anticipated. =D
Edit: Oh, also, ghosts can't walk through people, but it would be more like they're trying to walk into a statue rather than anything else. If someone walks into the ghost, they'll likely be shoved backwards until they're out of the way. If a ghost is stepped on, and there's nowhere that the ghost can go that's not solid matter from their perspective, then the foot goes right through their chest and they probably die. I think the scale of pain to death would be regulated by speed and mass of whatever enters their space, or something. But yeah, everything is moving statues that push ghosts around, and if ghosts can't get out of the way, they're in pain or they die.