Solo mentioned Shadow spells in the OP, SorO. In the PHB, most of the shadow spells are the shadow conjuration/evocation/shades line, and the other three (project image, simulacrum, and shadow walk) are not something that can have an effect while being believed that they can't while being disbelieved. Shadow conjuration/evocation, et al, is what the debate is about. The 20%+ reality of the shadow-stuff creature/object is what the debate is about, too. Can the caster choose to forgo his will save to disbelieve?
Regarding RC's wording alteration, consequence =/= 'always bad stuff'. It still allows you to choose to forgo a save.
Regarding the RotG articles, and the PHB quotes, they show how to define and adjudicate interaction, as well as define irrefutable proof. They say nothing about whether irrefutable proof removes the save, or makes it an automatic success. If the former, then the caster can't treat his shadow conjurations/evocations as completely real in regards to himself. If the latter, however, then the caster may choose to forgo his save, even though it would be automatic. He could shadow conjure a mount and ride it away, or walk on that bridge, without either having a chance to fail him.
As I was re-reading through the articles, I remembered a fun idea I'd had. The rules in the PHB, and the examples there (only) suggest that unless a creature takes special actions to inspect or muck about with the illusory floor, they don't make a save to disbelieve; they instead just assume all is as it seems and walk on the illusion. If it's a shadow illusion, then it affects them normally.
If this interpretation is used , then it would be hilarious to make a dungeon in a chasm or some-such using shadow walls of stone. Anyone inspecting it would need to fail a saving throw or fall X% of the time. And then, so would the rest of the party, as they'd have just seen something really strange as the rogue fell through the floor he was checking for traps.