Q1: I have multiattack +2 and a 10% crushing blow affix. Does this mean that each time i press the attack button I have a 3x 10% = 30%chance to cause a crushing blow. (Disregarding pierce and other effects. Is this correct? If no, how does crushing blow work?
If the multi attack is not being used as a multiplier, then yes each attack will be physically made, and will have a chance to cause CB.
Q2: Is crushing blow considered a proc? What targets does crushing blow work on? I noticed it did nothing against a mythic enemy.
It's considered an on-hit effect. There's a formula for how effective it is on rarer enemies, which i think ( off the top of my head ) is Actual % Damage Taken = ( Crushing Blow % Damage Taken / 100 ) ^ Rarity Level ( 1 for normal, 2 for magic, and so on ).
Q3: I have a multiattack +2 affix and a +2 extra attack affix. I tested it and when I shot there were ALWAYS exactly 3 arrows coming out, even though the number should vary between 3 and 5, depending on how many extra attacks i get. Is this intended? How does the interaction between multi- and extra attacks work?
Some of the extra projectiles might not be showing due to overlap, but they should still be working
Q4: Can multi- and extra attacks cause crushing blows or proc storm, earthshatter, comet etc.?
Extra attacks do not cause procs, procs are only on cast
Q5: how does pierce work for the bow? The tier 19 mythstone has the effect +2 pierce on armor. How does it affect damage, does it proc proc effects, can it cause crushing blows?
Pierce was forgotten about in the last patch, and still needs to be fixed Currently it does nothing.