Do I have to complete the Harbringer quest chain on each character in order for that character to participate in the Prepatch event on the 30th? Or is it account wide.
It would be kind of ironic if you had to do the Harbinger on every character just as they introduce so much account-wide sharing, wouldn’t it? 
I don’t have an officlal answer, and I can’t find one, but I have done it only on one character, and I will be seriously peeved if I have to do it … umm … 22 times!
From my understanding of their post, you don’t need it to participate in the event, but you do need it to do the quest chain that will open up Tuesday. (So, you only need to do it on characters you want to do the next bit of story on.)