Exiting via triggering an Esc press seems fine too.
I have a new issue with both methods however. Sometimes the conditions for my plugin have not been met (a user has not pre-selected polygons, for example) and the activated() function of MyPlugin …
Was lucky enough to accidentally find the answer in this thread! I simply call
pya.MainWindow.instance().menu().action("@toolbar.select").trigger()
to select another toolbar menu (the "select" tool incidentally), which un-press…