So to finish the Mysterious Box and get the Black Armory key I have to do Leviathan first, is that correct? If so then I'm going to have to try and meet up with some people to get that done or I'm hosed.
Nope, just farming the watchers in the Leviathan Underbelly. Like I said, it can be done solo in 10-15 minutes.
The Engine Room is the quickest to access as its the first room you get to when you enter.
https://duriandata.com/destiny2/raid-ninja.jsp