WR - false start vs. illegal motion vs. illegal shift

[stegenref]

I had a game last night where a wide receiver (on the line) started moving before the snap and actually crossed the line of scrimmage before the ball was snapped. I blew my whistle, threw my flag, and signalled the white hat that I had a false start. We went ahead and enforced it, but at halftime we talked about it some more. He told me I called it correctly, but he said if the receiver had not crossed the LOS, it would've been a live ball foul for illegal motion / shift and that case I should throw the flag but not kill the play by blowing my whistle. The Umpire told me that only interior linemen can false start.

So what is the difference between a false start, illegal motion, and illegal shift for wideouts and / or those in the backfield?