Hi,
Well, at first glance, based on the pictures, everything seems mostly okay, weird situation 
Perhaps two ideas to experiment with (if you want), but they might not solve the gurgling.
You can replace this O-ring with one from the spare parts kit. This might be a solution if the liquid is leaking down into the air channel and the current O-ring has become a bit “flattened” due to years of storage. If it still gurgles, you can swap it back, it costs nothing.
Maybe spread the cotton out more. Based on the picture, it seems to me it’s concentrated a bit too much in the middle, leaving room on the edges for leakage.
And a question: is the build deck (where the O-ring is) properly screwed onto the deck “base”? If you unscrew the 510 screw, you can then unscrew the top and bottom parts of the deck; maybe it wasn’t screwed on properly (no need to overtighten, but it should be securely fastened). Oh, and there’s also an O-ring inside the deck “base,” you could check that too.
I’ve encountered what you’re describing myself many years ago
The 510 screw was loose for me too, TC wasn’t working because the resistance was jumping, and several people on foreign forums mentioned this. I think this is a weak point, because if you tighten it too much, screw it in all the way, then it will be almost flush with the 510 thread, ruling out mechanical mods and some mods don’t handle such connectors very well. You need some middle ground, not too loose but still sticking out a bit. However, it’s still a good RTA.
But I don’t think this has anything to do with heating up; that’s more likely due to the clapton, as others mentioned before me: a large mass of metal, slow cooling down.