1. Alice is standing in a sealed corridor with a light on the other end. Bob the Illusionist, standing at the other end of the corridor, casts a Minor Illusion (or Silent Image, or something) to create the image of a wall that covers the entire cross-section of the corridor. Does this mean that Alice is now standing in darkness because the wall blocks the light?
Yes. Alice is now standing in darkness unless she perceives the wall to be an illusion. Casting a shadow (blocking light) is one of the primary features of a physical object's appearance. If illusions don't block light, any shifting light source (such as torches) will give away the illusion. Players will quickly learn and start testing for this and I would submit that asking for an Intelligence check while doing so makes the PCs seem like idiots.
2. If not, then consider a more normal case where Alice is looking at Bob in a large room and Bob casts a minor illusion of a wall in front of him to give him cover. Presumably in this case the illusion WOULD block the light that's reflected from the light source off of Bob (if the illusion generated its own light but didn't also stop the light reflected off of Bob, then Alice would just see the wall superimposed on Bob (and the background), since she's seeing both the light from the illusion and the light reflected off of Bob. So does that mean that the illusion can "tell the difference" between light reflected off of Bob and light coming directly from the light source, so it will block one but not the other?
I've read this four times and still don't understand it. In any case, the wall doesn't give cover, but it can give concealment. It looks like a wall to all concerned, except for Bob, who knows it's an illusion an can see through it.
3. If so, consider scenario (1) again but this time, the whole corridor is lit by a light source that is far away, and whose light is reflected off, say, a disco ball at Bob's end. Thus every ray of light that gets to Alice's end of the corridor (without the illusion) was reflected off of some surface at Bob's end. Does this change the answer to question (1)?
No.
4. Can you make an illusion of a mirror, and will it reflect light normally? So you could make an illusionary mirror to peer around corners?
Sure. With minor image the mirror would be stationary.
5. If illusionary objects don't reflect light normally, then the only way illusions could work at all is if they *gave off* light that matched whatever a viewer would actually see when looking at it if it were real. Under that interpretation an illusionary mirror placed across a corner would still show what's around the corner.
True. But minor image states that it doesn't create light so it either REFLECTS/BLOCKS light or it's TRANSLUCENT.
6. Another interpretation that I've seen is that the illusionist decides what the mirror shows - i.e. the illusionist is creating the image. This would imply effectively that (even in a normal case) the illusionist is figuring out what the illusion should look like given the ambient lighting and creating an illusion that shows that. (In other words, the illusionist has repurposed part of his or her brain as a GPU.) Note that this would mean that the illusion would still need to change as ambient light changed (and the illusionist would have to be initially aware of the positions of any light sources, especially if the object the illusion was being made of exhibited high levels of specular reflection).
Ack! Needless complication and waste of time.
7. If you know that an illusion is not real (e.g. because the illusionist who made it told you, or because you saw something pass through it) but you didn't actually spend the action to make the check to examine it, is it still faint/translucent for you? (What if someone who claims to be the illusionist tells you it's an illusion, and it actually is an illusion, but the person who told you isn't the illusionist? If this counts as knowing, it would be easy to get around illusions by having your friend "claim to be the illusionist" and "tell you it's an illusion" every time you see something that might be an illusion.)
Both minor image and silent image have the following text: "If a creature discerns the illusion for what it is, the illusion becomes faint to the creature." So, yes, to the first question. If you see the illusion fail to support a proper physical reaction, you discern the illusion for what it is. Likewise, if you're in a party with Bob and he casts minor image like he's done dozens of times before, you know the illusion for what it is and it appears faint to you (as does Bob, who cast it).
In other circumstances about being told that something is an illusion, you get into matters of authority (does this person know what they're talking about?) and trust (is this person lying to me?), which will be case by case judgement calls. However, I'd usually rule that a check is necessary unless the illusion was cast by a fellow PC. If an NPC you meet tells you a wall is an illusion, you'd need to "check it out for yourself" with a roll or physical interaction.