Being this a Layout Option, this is global to a specific layout (and yes: it will be global for that specific layout for all the flows that that layout shows).
If you use local Note spacings changes (with the same setting) as suggested by @Lillie_Harris , you don’t need the global Layout option, so that you can have many different settings in the same layout and in very specific rhythmic positions/areas, being able to set a Reset Note spacing change at any point after the actual Note spacing change:
(Sorry for my ignorance of what a “modal diagram” is: I will let other users answer to the second part of your question.)
If you would like to share an example Dorico file, it would be interesting to look for possible alternative solutions (beyond the nothead sets method suggested by @Lillie_Harris ) if needed.