Bearings are "self pumping", they generate their own oil wedge and draw from the galleries where the oil pump is providing a "head" of oil. Thinner oil flows through the bearing quicker, so the draw-down rate on the galleries is increased, couple this with increased leakage from all of the fixed orifices, and of course this is why you observe lower oil pressure. However, high pressure at the bearing galleries is not required, this is why engines without oil pumps can still function. Back in the day we even had engines that were gravity oiled.
I wouldn't call it "mist", it's the spray off the rod bearings.
Viscosity is directly related to temperature, which can change the realm in which lubrication is taking place in the engine. If you are familiar with the Stribeck curve:
View attachment 278075
We have parts operating in all three realms and some, like pistons and rings, that move between them, going from hydrodynamic to mixed to boundary and back again, depending on where they are in the bore and what stroke they are on. This is why the anti-scuff coating on piston skirts wears off over time. Increasing viscosity can shift where in the Stribeck curve a component is operating, for a given oil temperature, as it increases the MOFT.
Hydrodynamic is zero wear, this is where bearings are supposed to always operate. The Japanese OEM's, focused on fuel economy, fitting wider bearings, to allow them to tolerate some of the super thin oil grades we are now seeing, as a wider bearing can sustain an adequate MOFT with a thinner lubricant, while maintaining the same load carrying capacity. They found friction reduction in the mixed/boundary realm in certain parts of the engine, enhanced by various FM's and coatings, this improved fuel economy, which is what drove the pursuit of tinner lubricants.
My understanding of the GM issue, at least in part, is that due to manufacturing issues with the crankshaft, some of these engines are having bearings operating outside of the hydrodynamic realm with the spec 0W-20, which leads to wear, and eventually failure. The heavier lubricant provides a greater MOFT, which may postpone the manifestation of this issue.