Interesting idea. One easy way to do it would be to have 2 different paragraph styles: First-Instance-of-Heading and Other-Instances-of-Heading.
But I would probably do it with a single “Heading” style followed by a Running Header text variable. I’m not sure what you mean by that being a problem with “Header 2/10”

Thank you for sharing that, Chris!
In general, I try to avoid those insane floating boxes in Word files… I’ve had nothing but problems when trying to import docs with those into InDesign.

Maybe try a different script? Perhaps the Javascript version instead of the VB one? Or one of these:
https://indesignsecrets.com/how-to-split-long-story-into-smaller-pieces-unthreading-middle-story.php