Hi Kadda,
It seems the process of crystallization out of the melt solution could have happened how you described. The platelets of mica (Lepidolite seems a good candidate) could have formed in-situ with the corundum simultaneously, in that portion of the stone matrix. In other parts where the different minerals are stratified, could indicate a change of solution chemistry, thermal or pressure that stipulated which mineral developed.
There are still too many unknown parameters to suggest an answer.
Curious if the tomography scans provided any clues to microscopic structure boundaries or was it more macroscopic in nature?
-Troy