As far as I recall, the CFM-56 had a propensity to exhibit different acceleration rates between engines on the same airframe. I remember seeing effects similar to your 'scenario 2' on a few occasions. When the engineers were informed, the response was along the lines of, "that's within tolerance".
The only way to mitigate this was to be absolutely sure that the engines were stabilised at the same RPM before pressing TOGA. As you already noted, the higher the initial RPM, the less the effect.
BTW, I had a low speed RTO in an A320 with IAE engines caused by the 'other fella' selecting TOGA too early and having an apparently uncontrollable swing, so it's not just a CFM problem!
I have never seen your 'scenario 1', where one thrust lever momentarily stops or retards.