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Your description is confusing due to lots of hand waving, so it is hard to be specific.  However, an isolated section should be separated from the non-isolated parts <i>laterally</i>.  It depends on the exact spec or certification you are going after, but usually 5 mm is good.  It can be more than that, like for Intrinsic Safety.  Check your spec.

With the sections isolated laterally, there is no issue about which planes go to which isolated sections.  Everything in a section (all layers) are either isolated or not.

Do you really need a power plane?  Maybe the motor section needs it for high current, but likely the rest of the board doesn't.  My first knee jerk reaction is that you could probably do this with 4 layers instead of 6.

Having a ground plane is good.  In this case, you'd have two ground planes both in layer 2, with the 5 mm or whatever gap between them.  When you have a good ground, then a power plane is rarely necessary.  Unless the problem is DC current capacity, good decoupling of power to the ground plane at each point of use gets you the low impedance at high frequencies.

Low impedance at low frequencies is about brute force DC current capacity.  A plane can help, but there are often simpler and cheaper methods that don't require an entire layer.