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I am embarrassed asking this because the solution must be staring me in the face, but how do I change a layer from 25fps to 12.5fps? ie put it on 2s?

you can't change the framerate of a single layer.
Only of the project itself (Project modify)

but you can adjust the frameduration for each frame of a selection of frames. ( set exposure)

The easiest is by using the Handy custom panel.
This has a row of buttons to set the selected frames on doubles(2) for instance. this is the best approach, leave the project at the standard video framerate and set the frameduratiopn on doubles using the handypanel.
tip: double clicking under a layer selects all frames.