[Sephiroth]

First of all, I agree with Culhwch about suspension of disbelief. I've read things set in worlds that really don't feel much like real worlds at all, and still thoroughly enjoyed the stories. Having said that, I am the sort of minority pain who notices and gets annoyed by little details.

I really like the idea, it seems like you've put a lot of thought into it already. I have a few questions:

Is the world the same size as Earth? Same density?
Have people evolved there?
How did they evolve, and how long did this take?
(What kind of star is it?)
(Does the planet receive about the same amount of stellar energy as Earth?)

A cooler planet (interior) would produce markedly less tectonic activity. Less tectonic activity would mean that continental accretion occurs more slowly, which would give an Earth-like planet of a similar age (to allow for similar evolution to take place) a far higher ocean-to-land ratio, such as that which you suggest. If the planet was cooler (and the same age), it would also be less dense, however, and gravity would be less, so that the topographic range would increase, working against your attempt to keep elevations down. The topographical constraints you have imposed are fairly extreme for a 'living' planet.

But I'm not an astrophysicist, and the more outlandish and non Earth-like the planetary scenario, the less qualified I am to talk about it, really. A real expert would be of more use to you. There must be ways to get around some of these problems that I cannot see.......