I put off doing this MT quest until later, but when I asked about advancement, the leader of the Morag Tong advanced me to Grandmaster in exchange for my letting him retire.

As a result, even though I've killed her, the old Grandmaster is gone and without talking to him, the journal won't show the quest as completed.

I know you can punch in a code to the console and show the quest completed, but I can't seem to figure out what exactly I'm supposed to type in.

ID: MT_DB_AldSotha
Index: 100

Do I just type in "Journal MT_DB_AldSotha 100" ?

That should do it.