I recall that there is/was a Windows mouse setting that made it easier to operate with a trackpad by making certain mouse clicks a one-click affair, like dragging and dropping, and selecting multiple things. That may be worth looking into, if only to standardize all your inputs. (However, I don't know if LabVIEW controls would "subscribe" to that setting.)
Another option I sometimes use is to keep e.g. a boolean array in a shift registers and whenever an entry is double-clicked I switch the symbol from blank to a checkmark or back ...
Here's a quick example for that. I even use strikeout font for unselected items. You can also play with bold/plain, or with the cell background color (be careful to sync with the selection color).
I guess there should be an event for "long-click" as often used on tablets. Is there an idea for that already?
A while ago I wrote some code that would allow item selection in a listbox, MCLB, or Tree by checking boxes with a single mouse click, or right clicking for select none/all. Here is the included example: