Illusions are tough to adjudicate and RAW only offers some guidelines - in general a good rule of thumb is to keep in mind the 'total area' that the illusion can encompass - interaction typically means 'trying to determine if it's real' - as others have said it's usually a standard action minimum inside of combat - although I'd say if the illusionist is causing the illusion to interact with something intentionally that should allow a save without the need for an action. The fun thing of illusions is (if you are creative enough, and the gm is game) the ability to combine spell effects, or even duplicate higher level effects with a low level spell - when trying to judge this type of use just remember an illusion gets at least one save - and more once someone figures it out (at a huge bonus) and can be auto-disbelieved with proof - so a 1st level illusion duplicating darkness might seem powerful - in reality it's not in most situations.
2) I'd say based on the current RAW and writeups on illusions - no.
3) Yes assuming the entire area is within the limits of the spell - the enemy shouldn't get a save until their turn when they should use a standard to interact - at least the first person to do so anyway. Only once someone saves and shows the illusion to be fake do the rest of them get the automatic save. The key here is that the effect isn't actually touching the enemy - if the illusionist forces the effect to be in contact with the enemy I believe they should get the auto-save.
Another question important to the ramifications of 1,2, and 3:
You are supposed to auto-disbelieve if you *Know* it's an illusion - in such a way assuming the caster adds 'tells' to his spells for his allies he can create fog clouds over the party that frustrate the enemy but don't hinder the party - same with wall spells and other 'battlefield control or obstacle spells' - note however if he creates a wall and the party goes through it - that should trigger a save from the enemy.
And, a final question; That's a tough one - RAW I'd say - yes but only within the area of effect of the illusion. So for silent image "four 10-ft. cubes + one 10-ft. cube/level " - in the case of a skylight depending on how high up it is most likely not - and in that case the light being broken up would be enough of a 'tell' to get a save automatically. In any case I think uses like that will require a bit of the GM's touch in what they will or won't allow - I'd most likely try to keep lower level spells from duplicating much higher level effects - so for silent image I'd say lowering the light by one level might be appropriate, and perhaps for Illusory Wall (as a 4th level spell) I'd let it block any light source. Both are figments, but in this case I go with the 'the higher level the spell - the more you can do with it' - to whit:
Paizo PRD says "Figments cannot make something seem to be something else. "
Paizo PRD says "When the spell is used to hide pits, traps, or normal doors," (Illusory wall - figment)
The rules are slightly contradictory regarding illusions - so I fall back on the idea that the higher level the spell - the better it is at what it can do - also in the case of the more specific the spell *or* description - the better the illusion is.
I like the thought that an illusionist who wants to make a wooden chair and focuses on it can make a chair so real that you couldn't tell it was fake after falling through it - while something big - vague - and without alot of description will correspondingly be vague and easier to figure out.