Mix from a Diamond is illegal because the call Centers Cross Run can not be done from a Diamond. This is because the call Cross Run requires that

1. those doing the Run action must move into a spot occupied by another dancer;
2. those dancers occupying a destination spot (for one of the dancers doing the run action) must move to an adjacent spot which was previously occupied by a Runner.

At C1, the caller could say Squeeze The Diamond to get the desired result.

At C4, the caller could say Phantom Thar Mix to get the desired result.

Ceder Chest Definition of Mix