Atmospheric pressure in an almost empty world with a specific habitable zone far above the ground
I'm currently working on the following concept of a world for a high fantasy setting:
* The world's core has a radius of around 100 km. Inside is a magical object that creates a large-scale (planet-sized) gravitational field around it.
* The core is enveloped in a mantle of around 10-15 km of water with no additional notable land masses, such as continents.
* The "habitable" zone begins at around 8.000-9.000 km away from the core. This is where humans and humanlike species live under conditions similar to earth's sea level (same air composition and pressure, gravitational constant of approximately 10, etc.) on magical floating islands.
* No other celestial bodies exist in the universe, as sun, moon, and so on are divine constructs (comparable to e.g. 710
<PERSON>' chariot from Greek mythology).
With the basics out of the way I'm wondering about certain properties (especially relating to atmosphere and pressure as these are some of the most difficult topics for me personally) this world would have and how they could be explained with something other than "it's magic"? I don't require any hard numbers, I'm just very curious about what parts of this concept could be explained sensibly with the help of science.
* How high is the approximate pressure inside the water mantle and below the inhabited zone. Could any realistic creature survive inside it? Could the water physically even be liquid?
* How large would the "habitable" zone be? Just as earth's atmosphere gets too thin for humans to breathe above a certain height, how dense would this much larger atmosphere be at bigger heights? Would the pressure fall-off be linearly compared to earth's atmospheric pressure (e.g. a constant factor of x times earth's pressure for any given height or would this factor be changing depending on height)?
This is my first question here, so I'm very sorry if I made a mistake while asking this question! Please tell me if I did so and I'll try to correct it!
Thanks a lot in advance!
Edit: I forgot to mention that the magical object at the core generating the gravity creates it in a manner as if it was an extremely high amount of mass condensed into a small point.
I also went a bit overboard with the block of questions, sorry. I would summarize them as "Given the setting above, how would pressure affect habitability and atmospheric composition at different altitudes, assuming otherwise realistic physics?" 710