Courtesy staging - both top bulbs on before either can turn the stage bulbs on - is the rule most places, but not all.

Over the line burnouts are usually legal for all or legal for none. Sometimes door cars with front brakes will be prohibited from going across the line. Depends on the class, track, sanction.

Rolling across the line during a burnout may be allowed. I would think that stopping behind the line after the burnout, then rolling through the beams and backing up would be frowned upon.

Different reasons for backing into the prestage and stopping. Notice that they usually just backup far enough to turn the stage bulb out, Now you know a just bump will put the car back in the stage beam. Easier to get it in the same spot everytime than bumping in through both bulbs normally. In top bulb/pro tree racing, the race really begins when both prestage bulbs are on. If you can consistently get in first, set, and on your spot some people see that as an advantage.

Not picking up junk on your tires. Controlling the tempo of the starting line. Being able to get on your spot on the tree without having to bump into and through the prestage. When top bulb or pro tree racing, getting the top bulb turned on and set consistently the same everytime is more important than leaving on the bottom.