This usually means that for one reason or another the bearing races have become dimpled, and when the balls line up with the dimple[s] it acts as a detent. Backing off on the bearing preload might make the issue less noticeable, but is unlikely to make it go away. Going from caged balls to loose balls is another popular way of trying to prolong life in damaged headsets, the idea being to break up the alignment between dimples and balls.

Probably not, or only partially, depends on how bad its sticking. In a perfect world an overtightened headset would simply be smoothly resistant to turning whatever angle the wheel is pointing at.

Depends on you mechanical skills. If you're concerned enough to ask it's probably a good idea.