Well I'm sure I won't be able to provide valuable answers to all of your points, (or maybe not even answers @ all to some of them) but here are a few thoughts:
I would suggest doing as much as you can to fit into normal science before moving to handwavium.
Here are what I would consider your biggest roughly science based problems with this planet (to which I will suggest some solutions to after):
- Gravity... the gravity of a large body will, by reason of it's increased mass, be greater the greater the mass.This can pose many problems including limiting the size of the indigenous life, limiting the ability to leave the planet, and a few others I will mention below.
- The likelihood of development of powerful radiation belts (so long as there is a chance they could affect the surface or the peaks that the inhabitants live on)
Based on this article the affect might only make the poles uninhabitable.
- The affect of gravity on the composition and thickness of the atmosphere.
A higher gravity will attract & keep more gases, which will increase pressure as well as types of gases... which may make it harder for a simple enough atmosphere to provide enough of one particular type of chemically reactive gas (like oxygen), to allow living beings to breath it (handwavum may be required).
- Increased size, gravity, & proximity to the star could make it more of a target for collisions with comets, asteroids etc...
Some Possible Solutions
An approach that might solve a number of the problems is to have this planet located in a solar system that has a main sequence star which has transitioned to burning helium, and become a red giant.
This possible solution could work so long as:
- the planet is not too close to the star,
- the star doesn't expand too much,
- and that there was enough of a rocky core initially in the planet before it became a gas giant
So Given the above requirements here are the ways this proposed solution could be utilized, (with some handwavium, but not too much, applied):
- The risk of celestial bombardment would decrease because the incidence of large non-planetary bodies should decrease with the age of the solar system, the increased circumference of the star would eat most that formerly came into the inner solar system, and a gas giant has the potential for a number of moons and/or larger moons, that may help protect it.
- Through the explanation provided here you could (with a little not-quite-scientific explanation) provide for the toxic oceans as being the remaining heavier atmospheric gases or atmospheric condensates (such as acids).
This would allow you to also have the reasonably normal atmosphere, (water rain, oxygen-nitrogen atmosphere, etc...), while still having the oceans. It could also explain some smaller bodies of actual water above the level of the toxic oceans.
- Using this solution of starting with a gas giant that is then stripped of much of it's excess atmosphere, you could also explain it's large size as, generally speaking, larger planets become gas giants.
- This would also likely give you a smaller planetary mass than jupiter to avoid the problems of excessive radiation and gravity on your planet. I would suggest shooting for maybe 20 times the surface of earth instead of the 'surface' area of Jupiter 120.
While I suspect it's likely you are attempting to take into account the fact that denizens of this planet only live on mountains and highlands of the rocky core, due to the large ocean, while still leaving enough habitable area for them to live.
These mountains and highlands really amount to islands. If you remove the largest continents, increase the size of the remaining islands and multiply that surface area by 20 I suspect there would still be plenty of habitable area.
Interesting idea, and I hope this helps.