Thanks both of you 
I found that, when importing the alembic as a Geometry Cache, it doesn´t bring a second UV set, so my Z component was the same as the X component, and that wasn´t correct…
I finally got it working remaping my values to a 0-1 range and storing the “remap multiplier factor” en the alpha channel, so I can feed the PreviousFrameSwitch correctly. All this as Vertex Colours.
And, as said, works! 