Hi, I think I can help answer your questions.
I've personally have owned the Madagascar cage and the Tall Sturdy, also made bay HQ which is the same size as the Congo only has 6 extra windows on the front that have to be secured with zip ties and does not include shelves and ladders. These are well made and sturdy cages that have large doors making it easy to access interior and add/remove toys
/accessories that are large, such as the wheels
The difference is mainly the "foot print" and overall space, so size really. The Madagascar has a metal drop pan, while the Congo is plastic. The bars between the large front doors I believe are a little different. I like the vertical bars between on the Madagascar, but prefer plastic pans as in the Congo.
Yes, I've heard/read that ladders can be a hazard, and really they don't need them. After learning they may be a hazard, I removed them from the cage and have never used them since.
On the Critter Nation, I have not owned that cage and I believe it has a larger foot print than the Congo sized cage. I've read great reviews on them and yes, the center shelf can be left out when assembling, creating a large space for your gliders. You do have horizontal bars, which may be more easily climbed, but in reality, my gliders have never had a problem climbing vertical bars and I have plenty of varied rest areas and climbing options that give their grip a break, the climb vertically when they want to, not because they have to. However, the doors on the CN cage are amazing for cleaning and access, but you will have to be very careful not to allow gliders to escape while opening the door(s). I also like to use silos instead of water bottles and I also use them at time for the staple nectar portion. They do not work with horizontal bars, so it rules that cage out for me personally, but that's my preference.
I would just avoid the PP cage all together, to me the dangers of the pvc coating in the past just make me nervous about taking chances. The three other cages you've been considering have been used by many with minimal to no complaints.