I believe you're correct there on both counts. The inert gas gradient is already maximized when breathing O2 so added pressure from depth doesn't benefit the offgassing much, other than helping reduce bubbles.
Angelo - I learned that offgassing is proportional to the difference in the tissue inert gas tension and inspired inert gas tension. Your ambient pressure doesn't affect the tissue gas tension (i.e. you don't multiply by the pressure). The tissue gas tension is just a pressure value representing the actual pressure of gas absorbed inside that tissue - that is the basis of most decompression models.
You would only multiply by ambient pressure just to determine the partial pressures of a gas when you know its fraction (e.g. 79% N2 at 2 atm = 1.58 atm), but that is not needed in this case.
Because tissue tension is what it is, just a pressure value (e.g. 2.3 atm), then in the special case of breathing pure O2, where the inert gas tension is zero atm regardless of depth, then the rate of offgassing is the same regardless of depth.
There are other biological factors, such as elevated O2 PP being a vasoconstrictor, so the rate of offgassing could be in reality slower at 6m than at 3m.