On the topic of the Ascians having "failed" in the Dark Future timeline, I tend to think not. While we know that Ascians can make mistakes, it's pretty likely they'd err on the side of caution when it comes to the Source itself. Emet-Selch was aware of the properties of Black Rose, and seven times familiar with the amplification of devastation a Rejoining brings, and didn't seem even remotely worried. Not even a hint of, "Eh, we should be cautious in its deployment" or anything like that. And when he learned that the Crystal Exarch had come from such a Dark Future, he wasn't even REMOTELY interested in discovering WHY the guy had come back, just that he was able. He was completely confident that the Dark Future was one the Ascians didn't need to be worried about.
It's been my theory for a while now, that the sacrifice the Ascians wish to make of life on the Source to bring back their comrades can be fulfilled just by having an intact Lifestream. Actual plants, animals, and people aren't necessary to that goal at all, so long as their Aether is in the Lifestream - and even if Light-stilled Aether is unusable as a sacrifice (this is unclear - we know it's unusable for a Rejoining, but we don't know whether it's unusable for a sacrifice to Zodiark), plenty were dying from violence and starvation, rather than Black Rose, and would have wound up in the Lifestream instead.
Oh, I very much agree that it will probably be a one-time deal. The thing is, though, now that we know that it's possible, WHY is it a one-time deal? Especially for the Ascians? This is what makes it hard for me to suspend my disbelief. FFXIV is now a world where traveling to the past and altering history is something that's possible. It may be difficult, but that is irrelevant, especially when immortal beings are involved. Anything that can be one once can be done a second time. ANYTHING. One-shot components can be rebuilt. Lost technology can be rediscovered.
The writers don't seem to understand just what "immortal" really means. Immortal means that Elidibus could REBUILD the Crystal Tower AND Alexander AND Omega, by hand, out of Popsicle sticks and duct tape, and cobble them together into a new time machine, over a period of countless eons. He has the time! However unlikely an event might be, when you approach infinity the odds of it happening (or happening twice) approach 100%.
It would have been far better to keep the entire concept of being able to change the past an impossibility. They should have kept to the same time travel philosophy they used for Alexander: when you return to the past, the best you can do is participate to help bring about events that have already happened, with those events having an unchanged outcome.