I'm starting to feel like this Temple is where player choice goes to die, actually. First it was Leliana, now everyone seems to know about it, regardless of if you killed the only people who would have told anyone where to find it. If they make the Hinterlands into Elflandia, that's just another player choice disregarded, because it should only have happened if the HoF was Dalish and either dies or specifically requests it, which won't always be the case.
I think it's intended to give some "religious significance" to the player surviving the cataclysm, but I see what you mean. However, this has happened before. Anders and Justice still meet, even if Anders was never recruited and Justice was killed in the Dragonbone Wastes. No explanation is given to address how this is even remotely possible.
That said, I do wonder how this will be reconciled for the players who sided with Kolgrim and allowed the Dragon Cult to grow in prominence, especially if the dragon is still roaming around free.
As for the Hinterlands, I suggested that as an alternative for players who have performed the Dalish Boon with their Warden. However, I'm concerned that the Dalish Boon will receive the Magi Boon treatment, and that a flimsy excuse will be given to say it simply didn't work out, so that the area is basically the same, even for the players who specifically requested the People to get their own land.
Beyond that, I don't really have any issue with your scenario regarding being sent there to interview the alleged shade of Shartan. It's as good a speculation as any. I'm trying to remember what the Guardian's test was, aside from the obligatory 'fight yourself' battle, since that would obviously be an impediment to anyone trying to get further.
Correctly answering questions from the apparitions, fighting your own doppelganger, and trusting your friends to establish a bridge to cross.
To be honest, I would be surprised if any of us really gets to have the larger effects we're hoping for, unless BioWare has decided to toss player choice out the window completely and go with their own canon from here on out. There are just too many potential world changing states.
Yeah, I'm cautiously optimistic about the Magi Boon even being mentioned for my run, but I wouldn't be surprised if it's ignored entirely (like it was in Awakening and Witch Hunt). I'm curious about the Dalish Boon for players who chose that boon and expect it to be addressed in their run, since the US Ending for the Dalish Warden is Bioware's canon for players who aren't importing their own storyline into Inquisition.