So the mask size and position should always be the same, but the map inside the mask can scale and move?
I believe you can do this much easier. Put the map on the bottom layer with scale rate=100, put the mask sprite on the layer above it with scale rate=0. Set blend mode=Destination In on the mask layer (not on the mask sprite!). Also set Unbounded scrolling=yes for the layout and scale rate=0 for all other layers.
You should be able to scale the bottom layer inside the mask.