Well, first, this is basically just a semantics argument, so don't get too hung up on it. It won't change how you call the fouls.
For me, there are 2 categories of Illegal Motion:
1) WR "shaving" his motion toward the line of scrimmage. If it's very abrupt, we convert it to a false start. I would also put your example of the QB in motion toward the line in this category as Illegal Motion.
Any other foul that's motion/shift related, I call an Illegal Shift.
Agree, an all to often attempted deception (to draw the defense into encroachment): After routinely going in LEGAL motion during a contest, by a back taking one step forward, then pivoting either way and going in motion, often
( in critical, short yardage situations) the routine changes where the back
EXPLODES (or otherwise exaggerates his "one step forward" (ostensibly, to draw the defense into encroachment). When/should the defense fail to respond, the back simply continues to pivot and go in motion.
99.99% of the time, this is a planned procedure to sucker the defense into encroachment, and should draw an immediate whistle and flag for the
moving back's FALSE START. An acknowledging brief (unspoken) look directly to the Offensive Head Coach, may serve to reduce the probability of this intentional (but unprovable) deception being repeated.