I'm curious whether the effect of Marwyn causing a used plot's "When revealed" effect to happen results can impact characters that are immune to character abilities. Do you resolve the plot exactly like if it was revealed per the framework action or since the source of the effect was a character does it potentially resolved differently? Like would that mean that a Marwyn-triggered Valar wouldn't touch The Red Viper?

It is Marwyn's character ability that is actually responding at that point, not the plot itself. It is like a copy effect in that regard.

So if the card is immune to (triggered) character abilities, it will be immune to however Marwyn's ability resolves (as defined by the selected plot effect).