And thatI don't remember seeing anything that said specifically Mission and Zal, just 5 OR toons. It should be possible to use the new OR toons to do the event, tho I suspect you won't be able to get any of the new ones to 7* before the event rolls back around...
As seen above by following that link. He is a heros journey style release. This means you need the exact set of toons required.
Legendary releases are only faction limited, which is what you are thinking of.
...qutoes from moderator.The difficulty of events doesn't change.