I have been thinking about this too. Corypheus seemed to gain a fair bit of knowledge from interacting with the orb. He knew about the Temple of Mythal and the Well of Sorrows from consulting it. I also wonder if that is how he knew about the vallaslin being slave markings. His era was a very long time after the raising of the Veil so it seems doubtful it would be something he knew from Tevinter history, seeing as no one from Tevinter has mentioned it before. His aim seems to have been to enter the Black city and assume the throne of godhood but was it also to tear down the Veil completely? Did he see in the orb that it had been used to construct it? What did he think he was going to get from the Well of Sorrows?
I think the spirit in the Fade saved the Inquisitor and that it could have been a faith spirit, although that begs the question, why didn't it save Justinia since she was the one of faith you assume it was watching? Alternatively, could it have been a Hope spirit? These are meant to be very rare and powerful. That could account for it helping the Inquisitor because entering when they did gave the hope to Justinia that Cory could be stopped (not from killing her but from completing his plan), then them getting the anchor gave everyone the hope that the Breach could be closed and later that Corypheus could be defeated. The song that Giselle leads everyone in is a song about hope.
Strangely enough, my Lavellan initially believed that if there had been a divine being behind him in the Fade, it must have been Mythal (since he had her vallaslin) and thus an indication that his gods were returning to the world (ironic) and when Solas told him about the elven origin of the orb, he felt even more convinced that it had been directed to him as the rightful bearer, whilst Cory was the true thief, but I doubt that Flemeth had anything to do with events, even though she did know about Solas handing over his orb to Cory.
In the dark future I wonder if Corypheus reached the Well of Sorrows and got Calpurnia to drink from it (I think that would be his default choice over Samson). In which case, did he get any information from it? What happened to Flemeth in the dark future? Was she just holed up somewhere waiting for the merging of the material world and the Fade to get to the point where the evanuris were released? Or was she just cursing Solas for his stupidity because this wasn't the way it should have gone?
Most of Corypheus' knowledge about the elves probably came from Dumat. He did know about the Well before getting to the Temple of Mythal. Which is why he had Samson or Calpernia ready. I doubt he knew anything about the orb aside from that he held immense magical power. He probably figured it was like the somnoborium in Tevinter. In fact, the somnoborium were probably based off of the foci orbs. Cory just wanted to get back into the Black City and claim the dark power that was there, thinking it would make him a god.
Probably because Justinia wanted the player to survive. "Run while you can, warn them!" She knew she was toast. She knew the player needed to survive to warn the others. Yes, it could have been a Hope Spirit.
My Levellan thought she was saved by Falon'Din, since she has his vallaslin and because he's the "Friend of the Dead" and I think he is also connected to the belief in "fortune". Same with her getting the mark. After the Spirit she thought Falon'Din sent it to help her, and after Solas she believed that events were arranged by Falon'Din to where the Dread Wolf's plan will ultimately backfire on him, because now she knows that Falon'Din is alive somewhere and can be released (headcanon required, obviously).
Flemeth and Mythal probably thought that Solas would just wait (like a smart person) until he was strong enough, and then tear down the Veil.
Nope, Samson is the default choice. Cory's memory crystals at the Temple of Dumat state that "Samson has failed, but Calpernia stands ready". So Samson was always his first choice, while Calpernia was probably just meant to handle some of the Venatori issues (regarding slaves). Plus, Samson is picked to drink from the Well if you sided with mages, even though Calpernia is still around.
Cursing Solas, most likely. She clearly made no attempt to rescue him if he is captured. I suspect Cory was running into problems with the evanuris. I'm not sure if he got the Well's knowledge. Maybe it wasn't as effective with Samson, or maybe Mythal can choose who gets it. The Inquisitor who drinks can be in the Well's "world" and ask things before getting any answers, and then the Inquisitor has to agree to the compulsion or demand answers. So maybe there is a consciousness that can choose to reject someone who applies for compulsion. Then again, the Well may have granted the compulsion thinking that the person could be used against Corypheus, in which case Cory could control Samson with the red lyrium or Calpernia with the binding.
But yeah, Flemeth, Morrigan, OGB Kieran, Mythal, etc. They were probably all pretty powerless against Corypheus in the bad future.