I don't think the production of CO2 drives out most of the CO2 during fermentation. It's the yeast that is consuming it. The rate at which any gas permeates during a membrane is determined by the gas concentration (to be precise partial pressure) on either side of the barrier and the barriers permeability for that gas. You could have 100 psi CO2 on the other side but the O2 diffusion into the vessel is not affected. At least that is my understanding of the mechanisms involved here.
So you should move the beer once fermentation is complete. But don't rush it too much. Let the yeast truly finish the beer and the little O2 that the beer will pick up likely helps its aging process since stong dark beers seem to benefit from a little oxidation, IMO.
The solubility of oxygen would be affected by the partial pressure of the gases/liquids inside the fermenter. But you're correct that diffusion of air through the plastic would be occurring from the beginning.
If oxidation from air is the big concern couldn't you just immerse the whole fermenter in a water bath? Then the diffusion would be much lower (or nil) since the solubility of oxygen in fresh water is 9 ppm at 20 C.