If it specifies a number then you have to destroy/discard that number, if you can't meet the critera then you have to take the other option. If it says discard or destroy all then 0 is a valid number if you don't have any of that type.
In the scenario you listed you'd need to destroy all your cards in play to meet the conditions, this can be as little as 0 because it says all.
It's like Maintenance Level -- you can shuffle your trash into your deck to destroy it even if you don't have anything in your trash.
Yes, but if you have no cards in play and two or more cards in hand, you, have to do the discard.
Incidental Contact: does this mean if KNYFE hits them and then say Mr. Fixer redirects his shot to a villain that has already been hit then KNYFE can hit them a second time, a second time?
Yes
Caliginous Form: Let's say Twist the Ether is on the bad guy, Shielding Winds is in play, as well as Atlantean Throne Room.
Villain attempts to deal 4 damage to Writhe, Whether or not Visionary reduces or increases the damage Shielding winds makes it 3, then throne room makes it 2, and Caliginous form makes it 0.
Would it still redirect, giving Visionary a chance to change the damage to 2 damage? (4+1-2-1=2)
If the damage reaches 0 and isn't part of an irreducible damage attack then nothing is redirected.
Natural Born Vigor: If Naturalist is down 3hp, he can use vigor to heal to full and it won't be destroyed???
Wow.
Not in the scenario you listed which was "Destroy all of your cards in play or discard 2 cards".
It used to be that way - if you have nothing to destroy then you have to discard. Having nothing in play is having nothing in play, whereas having a hand of zero cards is still having a hand (so you can "discard your hand" even if it's empty). I think it was probably a couple of years ago that this came up, though - stuff gets changed around every now and then so maybe this was one of the things, or it's just another change that's only specific to the computerised version.
If one of the Sentinels is removed from game, then the Sentinels are incapacitated, and then a Sentinel is revived, does the cards that were removed from game that are returned include the Sentinel that was removed?