So:
What happens when you use shadow conjuration to duplicate a glitterdust if the caster (standing outside the area of effect) deliberately fails to disbelieve their own spell but an invisible wizard caught in the area succeeds in the disbelieve? Is the wizard outlined as per glitterdust or not?
You don't get to disbelieve your own spell. No rules on that that I'm aware of, but you cast it, you moved your arms, held stuff in your hands, waved your arms-you even sat in front of your spellbook and prepared it that morning. You know exactly what you created/cast. Disbelieving that you cast that spell reaches into philosophical levels of solipsism that give most of us headaches. So for the 1st example, the glitterdust, you don't get to disbelieve it-you know it's not real. As for the invisible wizard, he interacted with it and got a chance to disbelieve it and succeeded. It now functions as a 20% strength glitterdust. I'd say it negates his invisibility for 20% of the time, maybe roll at the start of each round.
What happens when you use shadow conjuration to duplicate the effects of mage armor? What if someone attacking a character 'protected' by such a spell disbelieves the shadow conjuration of mage armor? What happens if they don't? (How does the defender's armor class vary?)
A failure to disbelive treats the mage armor as present for appropriate AC bonuses (+4 armor bonus, works vs. incorporeal attacks). Successful disbelief would grant 20% of the effect; I'd still give them a +1 AC bonus (rounding up from the 0.8 they'd otherwise get).
What happens where an object created by shadow conjuration is used to block a gaze attack? (For example if a blanket created via greater shadow conjuration simulating minor creation is used to make a cotton blanket which is tossed over a medusa's head?)
Failure to disbelieve would, if this blanket was successfully applied, block line of sight and line of effect. I don't know how the medusa's gaze will work until the Bestiary comes out. If you have to see the medusa, then you're still up the creek regardless of the blanket unless it covers enough of her to make her identity doubtful. If it's an active gaze weapon, then she'd have to pull the thing off first. If she disbelieves, then there's still a 60% chance of success of the blanket keeping her gaze from working.
Even if you are in a position where you will automatically 'see through' a shadow conjuration (via some means such as true seeing) can you deliberately fail to disbelieve in it, if it would be beneficial to you to do so?
'Automatic' means just that. Choice isn't involved. You know something isn't real (such as in illusionary stone bridge) when use true seeing. Instant brain input: it's an illusion. Whenever you interact with an illusion, you get a save to see if you figure out it's a fake. Yes, you can deliberately fail a saving throw. But let's take that bridge. You touch it and you get a save. What are you really doing in the game? You touched the bridge and your senses tell you something isn't right. How do you rationalize ignoring your senses? That's up to you. Ultimately, though, this can lead to all sorts of insanity. You choose to ignore the fact that your fingers and eyes and nose tell you the bridge is a fake and walk out onto it, even though your potent spell tells you there's nothing there. And then you fall through the illusion. If you know the bridge is a result of a shadow effect, it still has some strength to hold you up regardless of your belief, maybe even enough to do the job.
Until one of the resident gods of knowledge appears, here's my take.
Lathiira:
Thanks for the thoughts.
If, when trying to disbelieve something which you believe to be an illusion, you can attempt to ignore sensory feedback, then wouldn't you be able attempt to ignore such input if you have a true sight running and want to 'believe' (and see) something your sight (modified by a spell) is telling you isn't there?Edit:
I'd still hold to the thought that 'automatic' means 'no choice'. The guy with true seeing best go and look away. The rest of the PCs might well be in trouble if one of them has to interact with the blanket as well, so I strongly suggest that the person who creates it do the blindfolding and then keep things simple.
And yes, I remember those annoying 2E Tome spells. Illusions can be aggravating enough, why reinvent those frustrating things?;)