Can weapons that hit multiple enemies with each hit (like 2H melee weapons,cannons and shotgonnes) which have lifesteal properties heal you for each enemy hit per click or do they only heal you for the primary target you're attacking?
I am pretty sure it is just for the primary target. I have tested this by setting Damage text setting to verbose and got myself a cannon on an engineer with +8 Health per Hit. Even though my cannon was hitting many mobs I get only one +8HP sign. I double checked my health globe too. It says the same thing. IT goes up by 8 every time.
So the short answer is you get only 1 +Health per shot/hit.
Otherwise imagine the cannon engineer exploiting a 4 slotted cannon with Blood Embers and blasting away at crowds... virtually invincible!!
I think so (75% sure)?
It is very hard to tell where the lifesteal I have is coming from. I get it from my pet, my wolf summon, my crits and my weapons. But as a melee char I have to be in the middle of a group of enemies to be dealing any damage and I cannot really see what is going on.
Crits can hit multiple enemies at the same time and I certainly get health from each of them.