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@doctormike, let me preface my response with what you said "I'm (also) not a deco expert.." so If I'm wrong here, I hope someone corrects me!
I don't think pressure itself is really the issue. Gases move into and out of tissues by diffusion. The bigger the concentration gradient, the faster they move. What pressure does is just change the concentration gradient. You on gas at depth because you are inspiring inert gas at a higher concentration than what exists in your tissues.
So in your two thought experiments, I think the answer would be no, you would not get bubble formation. Let's make a caveat for scenario number 1. I'm assuming there is a layer of gas above the liquid and that layer of gas has a PN2 of 0.79. Now in both scenarios, even though you are changing pressures, you haven't changed the concentration gradient. So yes, you can change the pressure, but now, there is no concentration gradient for the gases to follow.
Again, if I've really missed something here, someone correct it!
No. The CHANGE (specifically, the decrease) in partial pressure is relevant to decompression stress
Offgassing means that the gas is in equilibrium between tissue and blood, and then flows along a concentration (PPN2) gradient across the pulmonary alveoli and is cleared through exhaled gas. That gradient is dependent on the PPN2 of the inspired gas, so if you switch to a gas with less N2, it will happen faster.
In off/ongassing the tissue/blood and/or blood/lungs are NOT in equilibrium. A change in pressure across those barriers will cause the gas to flow. Gas that is flowing is not in equilibrium because the particular tissues on either side of the barrier are not static, they are increasing or decreasing in pressure. Tissues in saturation are in equilibrium even though there may be local areas where there is a constant switching in flow of gasses across barriers. Over time these average out to a constant pressure. The gradient is dependent on the PPN2 of the inspired gas and the ppN2 of the inspired gas is dependent on the ambient pressure.
How does the ascent in this dive compare to the ascent of a free diver?
That one's easy: in a freediver on-gassing is limited by the mount of gas in their lungs, so their max. loading should be negligible in terms of decompression models. I think.