Hey I was the other friend Cahliope mentioned. Might not be low enough; we’ve done it twice to verify, on horde and alliance. On horde, I was on a level 60 Belf I hadn’t touched since before DF. On Alliance, they were on a level 45 hunter. Both the max level characters we tested had finished the TWW campaign fully.
Step wise:
Lower level friend syncs party, max level character runs down to Khadgar in the portrait room (where he was for the prepatch event/starting the TWW intro), talks to him, and skips the TWW intro to be put into Dornogal. Immediately go back to Dalaran, have lower friend cancel party sync, and Khadgar phased in, right in front of us both times.
Haven’t verified this with the issues people are having with class hall progression, but it’s worked for us trying to turn in the luminous pearl quest to Khadgar in the Violet Citadel for the Underlight Angler.
Hope another attempt works for you!