-they are working on it. Some skill are reactionary (like the one you speak, where it work after you miss a hit), but right now they get highlined as every skill.
-because you're not supposed to do it. Granted, no skill is useless now. You'll use everything
-you don't have a delay. Maybe you are in a previous animation ?
-of course you do, since they hide, aka you lost target on them...can be annoying, but have it's purpose
-that's because you're pointing in an empty zone of screen ? Usually target don't wear off if you click on a window.
-it's called sub-target. Classic example, if you're fighting 2 mobs and want one to be sleep'd, you can cast sleep on the second mob without losing target on the first. On the other hand, you use weapon skills only on the mob you're fighting.
-as i said, it have it's purposes.
- I recall yes
-They can be interrupted.It's not very common, but happens
-mobs can be attacked from everyone, but only the first engaging take exp and loot. The guy you speak of is either doing you a favor, or a bot farming sheeps