Nerite lost his trapdoor today. What's going on?

On the 2nd of November my largest nerite snail looked like he was struggling to change direction on the glass. I went to do a chore for about 20 minutes, then went back up to check on him.
This is when I found him lying upside down on the sand with his trapdoor closed.
Fearing the worst, I picked him up to smell him but I smelled nothing and he was firmly shut.
I put him back, right way around, on some pebbles that had algae on them. I kind of hoped the presence of food would get him moving again. No such thing happened.
The next day another of my snails fell off the glass and was upside down with the trapdoor shut. I immediately removed all three and put them in a temporary home with a rock covered in diatoms (this has been there favourite food as far as I can tell) and seachem prime.
As soon as I moved them over, the second one to fall down was recovered and acting as always. But the big one had still made no progress.
I've been doing 50-100% water changes every day because there's no filter in the temporary home (my filters are all in use) and because they've been pooping like mad.
Every day I check on the big nerite and smell him. He never started to smell, but after the second day his trapdoor opened a little. He was sort of sticking out a little but nowhere near the edge of his shell.
Today I was doing their water change again and put them in a jug with treated water. I put the big nerite in a bucket with water because I wanted to check on him without freaking out the other 2.

The old water smelled a little for the first time, and it also had the most poops in ever. Like, where is even coming from?
I smelled the snail and still no smell at all. Not even the water smell, so I assume that was just the poops smelling. Pleasant hobby.

I'm still worried about the big nerite because it's been a week now since he fell of the glass and closed up. I read online you can shine a torch behind their shell to see if there is a heartbeat. Don't know if that's true, but I wanted to try it because I don't know what else to try.
When I picked him up out of the bucket, I heard something and looked down and his trapdoor had just fallen off.
He still doesn't smell, he didn't fall out of his shell and he isn't drooping out either.
What is going on?
Is he dead? Why doesn't he smell, if he is?
If he's alive what is happening to him, and will he live without his trapdoor?
What should I do now?
I've attached pictures below. First one is the first day his trapdoor opened slightly. Second picture is him lying next to his trapdoor in the bucket. Last ones is what he looks like now.

 

The trap door dose fall off some when they die, the fact theres no smell is because he's not started to decompose. We see this a lot and yet still know very little about them

 

Also, if you kinda freak everytime you see a snail on its back, this will be a stressfull hobby for you! That is a very common thing amongst my mysteries and nerites. They can generally right themselves as well if they choose to

 

The trap door dose fall off some when they die, the fact theres no smell is because he's not started to decompose. We see this a lot and yet still know very little about them

So he's definitely dead then?
Do you know if he died the day he fell off or in the week since?
Everywhere online made it sound like they smell instantly. Is it possible that he isn't decomposing yet if it's been so long already?
Thanks for your answer.

Also, if you kinda freak everytime you see a snail on its back, this will be a stressfull hobby for you! That is a very common thing amongst my mysteries and nerites. They can generally right themselves as well if they choose to

I didn't freak because they fell over. I know that happens and I know they can usually right themselves. What worried me was that they both did it for the first time in the matter of 2 days and I've never had it happen before. They had the tank to themselves, no fish, so there wasn't much to make them fall. Nerites have that strong grip. Also the fact they closed their trapdoors. If they were alright, they would be trying to right themselves, and so, open.
The two little ones are completely fine and living life, but obviously the big one is not fine.

 

Fishlorians quite often publicise their snails doing things for a week or more before finally picking them up to do the sniff test, he probably died in the bucket and you dislodged the trap door picking him up, he probably won't start to smell for a day or two
If you touch where the trap door was you shouldn't get a reaction from him, that's a sure sign he's dead
If you leave him in the bucket for a few days and keep sniffing him the very unpleasant pungent smell will hit you like a 10 ton truck

 

Fishlorians quite often publicise their snails doing things for a week or more before finally picking them up to do the sniff test, he probably died in the bucket and you dislodged the trap door picking him up, he probably won't start to smell for a day or two
If you touch where the trap door was you shouldn't get a reaction from him, that's a sure sign he's dead
If you leave him in the bucket for a few days and keep sniffing him the very unpleasant pungent smell will hit you like a 10 ton truck

Sorry for the late reply.
Yeah he'd been still for a week, when I posted. Sniffed him every day when I cleaned the water.
It definitely came lose when I picked him up from the bucket, because I heard it fall down then. It's strange it didn't drop when I moved him from the temporary home to the bucket though. It was literally like 1 minute he was in the bucket just so I could tip out the water without pouring him down the sink...
I'll keep smelling him for 2 more days, like you say, I'm sure I won't miss it if he does start to smell.
I'll give him a little poke tomorrow although I doubt I'll get a response. If he didn't respond to losing the trapdoor, I don't have much hope left.

What can I do with him once he's definitely dead?
I was kind of toying with the idea of putting him in a cycling aquarium for ammonia... Once his little body has decomposed or fallen out of the shell, can I leave the shell in the aquarium with the other snails as a calcium source for them? Any thoughts on this?

Don't want to be insensitive about the life of an animal, but I'd rather have his shell around than just bin him. He's been my favourite snail so far.

Thanks for your message

 

I wouldn't leave him in the smell when they start to decompose is something that seems to linger in your nose, just like when I feed broccoli the smell comes out of the tank and I think the same will happen here.

 

I wouldn't leave him in the smell when they start to decompose is something that seems to linger in your nose, just like when I feed broccoli the smell comes out of the tank and I think the same will happen here.

Okay thanks. I'll remove him so that doesn't happen.