well the submitted class is only added to the mcq-widget div so, although I'm no CSS expert, I think it would be difficult to hide the entire question using that class.

I'd be more inclined to try to use the completed class that's attached to the component div instead; almost certainly in combination with a selector that specifies that its ancestor must have a class of assessment - and quite possibly in combination with a selector that requires the component div to have a question-component class as well.

Equally I suspect you could write an extension that listens for questions being answered, checks to see if they are part of an assessment and then hides them via JS.

I actually did see your other post but I'll reply on that thread so as not to get things mixed up