High KV100 isn't going to cause harm - you think Nissan would show in the OM that a 20W-50 could be used if it "caused damage"? You're not understanding the graph. It shows that if the bearing clearance is too large than the MOFT decreases. This happens with any viscosity as the bearing clearance increases - even worse with thinner oil.
The worse combination is larger bearing clearance with thin oil. The lower the viscosity the more the MOFT drops off as bearing clearance increases. This is why the journal bearings ideal clearance is around 0.001 to 0.003 inch in that graph, where the curve is near max and pretty flat. Go back and re-read those posts. and go to that SubsTech website is you want to study journal bearings and how they work and react to viscostiy, speed and load.
The bottom line is you want more MOFT than not in journal bearings because that is the only thing keeping them in full hydrodynamic lubrication, and not wearing out because the MOFT is too small of goes to zero often. This is why going up at least a grade is benificial to give some added protection.
Well sure, going with a xW-30 over a xW-20 is going to give some more MOFT, which is what you want. xW-50 would give some more, but unless you're going to tack the Rouge then xW-30 is good for street driving if you wand some wear protection headroom. Who doesn't like headroom in things where you can achieve headroom?