I was at the local shop and was told these need to mounted off axis in the door, not a kick panel facing the center of the car. I could not find info online to support this. Does anyone know if this is actually true?
Technically speaking, kickpanels would be off-axis and the doors are on-axis. Given that those drivers have such a short Xmax (2mm), I can't imagine they'd be very good off-axis. MB Quart says "perfect for factory door-mounts, while custom mounts may require some experimentation".
Kickpanel mounts can sound nice, though usually require supertweets to level the stage (avoiding the sound rainbows).
Jonathan may be able to lend more firsthand advice, but it sounds like your installer is on the right track.