Is there any more summary of the War than the bare timeline?
Not really. A lot of the history of the War has been left intentionally obscure, both to provide for possible development/stories later on, and because the entire thing is supposed to be clouded in a certain degree of mystery anyway. There was few left alive who really know what happened besides Ellia, and she doesn't see too inclined to speak about it.
What are AEther Vents? What are Bells? Who used them?
Both the AEther Vents and the Calling Bells are
Artifacts of the Dominia Cabal, a write-up of which I have just linked. The AEther Vents are sort of the nuclear weaponry of the War, designed to relentlessly funnel pure AEther into whatever world they were detonated on, essentially tearing that world completely apart. The Calling Bells were created by the Mentalist, and used by the Cabal both before and during the war to control the mortal populations under their control. An exploration of those bells can be seen in this story:
The Deluge at CarakeraWhat was the Master's plan that he unveiled? What world was he planning to destroy?
To be honest with you, I'm not sure. Tevish may have to answer that one.
What were the Grail and Black Grail?
Planeswalkers, and their inherent pre-Mending powers, were a finite resource for the Cabal, both for the loyalists and the rebels. The grails were created in an attempt to rectify that by allowing ordinary mortals to become planeswalkers. The Mender created it. She had the best of intentions, but was unable to succeed in any real way. After the War began, the traitors usurped the project for darker means. I assume it was the Scientist (though I don't know that for sure) who darkened the intention of the grail and found a measure of success. Although it was not a true Spark, it did provide the mortal with immense power and the ability to walk the planes, but at a tremendous price. The black Grail figures into Tevish's story:
The Planeswalker's DaughterIs there a description of Sanctuary anywhere?
Unfortunately, we haven't gotten that far yet. Sanctuary was my brainchild, and so I have some thoughts, but nothing has been decided on yet. I envision it as a large world devoid of sapient (but not sentient) life that actively restricts and restrains magic. Planeswalkers are able to come and go, but working magic there apart from simply 'walking is next to impossible. I see it as a world of pastoral peace.
What is the Empyrean?
Empyrean is another world created in the crucible of other, destroyed worlds, but it was created in a much different manner than Sanctuary was. The development of Empyrean is still on-going in this thread:
The Work of Gods, though admittedly, development has stymied a bit. (Any help would be much welcomed!)
Is there a summary somewhere of the project and the ramifications regarding M:EM overall (such as this Calendar that gets mentioned, as though it's going to be our dating method now instead of AR).
The calendar was actually the genesis of this entire thing. We really need to pull the trigger on that. Basically, we need to decide where on the AR calendar ours will start, and how long our year is in comparison to a Dominarian year. A lot of this ties in to Zhiran, who is the original creator of our reckoning, and the idea I've been going with is that one year in Zhiran's calendar is equal to one year on Sanctuary...whatever we decide that should be.