I heard back from Jack at Edge Composites. The compression plug does rest on the top (cut) surface of the steerer tube, which is their way of guaranteeing that the expander is positioned sufficiently far enough down in the steerer. Then, as everyone has of course indicated, the spacer on top of the stem has to be sufficiently large enough to ensure that the top cap does not contact the plug (Edge suggests 10mm!). So I'm using the Edge plug but with a King top cap, so I can use a 5mm spacer.

As I previously mentioned, I think one thing that is somewhat unusual is that Edge (and others) definitely wants their steerer tubes to extend up beyond the stem a bit, necessitating a spacer on top of the stem (regardless of the type of expander plug). I see a lot of carbon forks, but I don't recall seeing spacers on top of stems before (maybe I've just never noticed!).