While i am fairly noob to ants, i believe the nest behavior you describe is fairly normal. The ants will seek out most optimal conditions, and those will each be differetn for eggs and larvae, pupae, and adult ants. With the eggs/larvae commonly kept in higher humidity ares than adult ants would hang out in. And pupae/larvae kept where it is warmest.
I notice my Pogonomyrmex. colony shuffles the brood around over time. Keeping the eggs on/by the water tower, but moving the larvae and pupae back and forth between the water tower and where the heat is generated(staying in each locaiton for long periods).
It's just a mini hearth, so only one small chamber but they still notice and make use of the temperature and humidity gradient across it.
So i'd not take that behavior as indicting anything is especially off. They just do notice and do take advantage of minor temperature /humidity differences for optimal brood conditions.
As far as hanging out at the bottom of the tube. I have read it can be a sign of a crowded nest when enough of the nest is seeming to live in the outworld/connecting tubes.
And just to say, neat setup you got there.
However never forget that all creatures run a "conservation of energy algorithm." They want to choose flat or downhill routes whenever they can, and may have just decided that part of the nest includes this lower portion of the tube as part of that.
If they just live there, then they don't need to make the trip down, only drag food back up.
So i'd imagine that too could be a factor on them in the tube.
All i know about outworld is just that the larger the space the more natural their behaviors can become. In a small enough enclosed space their sent communications wind up with behaviors a bit differetn from in the open wild as i understood it.
I have made sure to provide a fair amount of outworld space for even my small colony now only at about 40.