There’s no perfect solution, and mac users need to live with the legacy of single-button mice. So the ‘control’ key on the mac keyboard together with a left mouse click is equal to the right mouse click on other platforms. (And shift-left mouse is the middle mouse button.)
So yes, as long as both Qt and VTK both uniformly map control to the command key we just need to find a clean way to communicate that.
The cleanest I have seen is to just have multiple shortcut tables as needed for the platforms you support (e.g. here’s what Chrome does). That way you avoid the extra visual distraction of trying to put multiple shortcut options into a single string, particularly when there are multiple modifiers involved. So you get “⌘ + Shift + b” in the mac table and ‘Ctrl + Shift + b’ in the windows/linux table rather than the probably confusing “Ctrl + Shift/Ctrl + Shift b” in a unified table. So I’d vote for either separate tables or separate columns.