Having tried everything we could imagine from my first post, can anybody guess at why Finale software can correctly identify key modifiers with a menu item selected manually with a mouse, but can't detect them when a menu is selected with KM?
The Select a Menu Item action selects the menu via the accessibility API. So there may not even be a real “current event” at that point to process, since the accessibility API is not part of the event queue. If so, then Finale’s behaviour has accessibility implications which they may want to think about.
I revisited this issue (urged by the veteran Finale scripter) and your 3rd option WORKS! Turns out that Finale needs a MOUSE-UP WITH MODIFIER event, at least to pass modifier key information through to the plug-in (via PDK) with a menu. Several methods work to get to the required sub-menu item, but there must be a click on that sub-menu followed by a RELEASE with modifier.
This will be valuable information to a small band of dedicated Finale users who use KM for all their GUI needs. I've attached a pic of the macro and the macro itself - customised for one of my tasks.