With a scrum the ball cannot be put in until a signal from the referee. That is clearly stated in law.
A better analogy would be a tap PK or FK, not a scrum. At a FK/PK once the mark has been made (or indicated) the kick can be taken.
no, no, not at all -- in fact that's the whole point
a QTI (Quick-Throw-In) - is like a tap FK or PK ... the ball is not dead, the oppo and referee absoutely need to be paying attention.
a Lineout is a set piece. It IS like a scrum. It may happen at varying speeds but it can't happen while the referee is busy attending to something else.