Being level 85, I decided to start on Dread Wastes, as it's one of the maps I haven't completely tired of.
I accepted this Command Board quest to come to the Serpent's Spine and it turns out Bowmistress Li and her compatriots are phased.
When flying into Dread Wastes I can access them for a fraction of a moment before the phasing of the Gate, which lies in the Vale, takes hold.
I spent some time searching the Vale for a clone of Zidormi, either in the Horde's city in the Vale or in the Mogu'shan Palace, with no luck.
At level 85 8.3 isn't happening in this version of the Vale, but it has been shattered by Garrosh.
I thought that I had recalled seeing Zidormi along the side of the Mogu'shan Palace sometime in the past, as well as in front, near the memorial that was set up after Garrosh wrecked the place, but I couldn't find her anywhere on the Vale map. Good chance I did miss her somewhere but.. whew. I really did look!
Next logical step was, perhaps it was because I hadn't done the Temple of the White Tiger quest to open the August Celestial Gate to the Vale. ...Ran that real fast and the Bowmistress is still phased.
The only other option that I can think of is that I would need to open one of the other bread crumb lines for the Bowmistress (listed on her Wowhead page, in the comments section, by user Karadine).
I'll pick one and run the gauntlet until I'm sent to meet with Mistress Li to find out if that solves the phasing and share my findings (long winded as they may be). ;)
Odd that having a Command Board quest doesn't seem to bypass all this mess for this particular line.
Edit: Couple hours later and the character is level 90 with no need to stay in Pandaland, but at this point I just really want to know what the phase trigger is.
Edit x2: Dropped this quest and picked up another bread crumb quest: Better off Dread with no additional luck. I'm sorry to say that at level 93 I still have no idea what the trigger for phasing the Serpent's Spine is.
Rechecked the Vale for a Zidormi, sure I missed her somewhere, and am now satisfied that I haven't (at this level).
Sad conclusion, but I feel enough time was spent on a pointless curiosity.
If someone figures it out I'd love to hear about it! :)