how much movement can a flexible adhesive take?

It doesn't really take up movement; it absorbs any minute flexing of the substrate and the tiles and as your walk across them.

You should aim to have no movement at all in the substrate.
 
but a tiny amount of say a floorboard under the plywood is fine?
 
No - I would say that you'd be leaving scope for problems. Lift the plywood and screw down all loose floorboards.
 
The word flexible applied to an adhesive is a misnomer, as though its elastic, which it is not.
It really dries to a non brittle state, which means it will not crack easily,and allows expansion/contraction to occur, and tiny amounts of movement.
You want your substrate as solid as possible......
 
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