Hey! I got it sorted.

Here's the thing.
1 To use F5 to effect changes on specific chorus, say mute strings on Chorus 2, you have to select chorus first, then do the muting selection, not the other way round. Might seem obvious but I think I happened to do it the other way initially.
2 If you re-open the F5 dialogue box it appears the change hasnt been saved - but that's because the Chorus settings always opens on "All Choruses" by default. If you then select the chorus in question, in this case chorus 2, it then displays the relevent change.
3 The strings wont fire up again in chorus 3 without the "Return to Normal Command" This is also entered in the same F5 dialogue box under chorus 3. Both settings are saved.

Slowly getting there...