At any of those frequencies moving my head a bit to side to side or back and forward causes audible SPL shifts in both channels, all local interference effects. In general - I'm not sure how valid this kind of test is. In any case, the higher the frequency the more annoying it seems to get
Yep, not easy to do, but it can still be done.
2kHz is -3dB according to ER/MMM and is perfectly in between with ON/LW. Such difference should be audible so you'll be able to detect if you're hearing ER/MMM or ON/LW at your listening position.