Yes, I get the impression that Mongo as shown in this movie ( it was a normal terrestrial planet in the strips.) is meant to be a gas giant or something similar, Ming's Kingdom, or Mingo city is on the rocky core or central planetoid of the ''gas giant'' somewhere at it's centre.
The outer worlds or Moons of Mongo orbit above in the upper reaches of the gaseous atmosphere.
The imperial vortex is some kind of Portal for interstellar or interdimensional travel.
Everything will be OK in the end, if it aint OK,it aint the end.

Hawlkmen sky kingdom hovers over arboria. Flash suggests they make parashoots and float down to arboria.
Wiki says Mongol has gravity slightly less than earth, so the floating moon kingdoms I think are like the floating mountains in avatar. At least with aquaria.

It's a fantasy realm with no attempt to be physically accurate.

Yes it was seen in the original Buster Crabbe serial. You definitely see it in the original strips too. As for this movie, I think it is a planet, you just don't see it. Aura says every moon of Mongo is a kingdom.

I think there's an old black and white movie serial that shows it as a planet from space, but that obviously doesn't apply to the 1980 movie we're discussing here.

I got the impression that Ming's palace was on the mainland and that all the other kingdoms floated above it like moons. The "planet Aquaria" that Aura points out looks really bizarre, like four or five big floating chunks of rock. In the original comic strip all the kingdoms were spread out over the surface of Mongo. Its really strange to think that considering Mongo is a planet you never ever see it as a planet from space.