Referring to omnifocus, use a switch as multi-selection.
In my experience, I am used to it (only my opinion, without research), but just below the multiple selection there is a single selection, and they look exactly the same:
Same control, different interaction:
And on the mobile:
Observe how they should send comments to the user to ensure their selection on the switch:
It seems you have room for a row of check boxes, and it makes it clearer:
Question 1: Is it an error or a characteristic?
Regarding question number 1: if you have problems to know if it is an error or a characteristic, suppose that the user will not have more context than you.
You can try both options before making a decision.