Questions concerning procs

Hello there!
In preparation for my farm rogue that is being theorycrafted, I have a few questions about multiattacks, extra attacks, pierce crushing blow etc.

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?

Q2: Is crushing blow considered a proc? What targets does crushing blow work on? I noticed it did nothing against a mythic enemy.

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?

Q4: Can multi- and extra attacks cause crushing blows or proc storm, earthshatter, comet etc.?

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?