Not for sure if this is news, but I just noticed that Volume 3 is available for preorder on Amazon. Looks like it'll be out by April 9th, 2019. Oddly enough, I don't see it listed on Dark Horse's website.
I think I'm going to ask for Volume 1 for my birthday, since it's gotten cheaper. I saw it in the bookstore when I was browsing the other day, and I was surprised by how big and heavy the book was. I may do the same thing with 2 and 3--wait them to release and then drop in price. Depends on how much I want to read ultimania stuffs on XII.
In addition to the 25th Ultimania (which are mostly artbooks), I expect the 20th Anniversary Ultimanias should contain more information about FFI closer to what you find in a normal Ultimania-- they're three books covering FFI-12 and X-2, each focusing on a different topic (Character, Scenario, and Battle). They each have 15 or so pages for FFI. I don't own them so I can't tell you that much.
There have been companion books for the FF games since the original FFI. But they were basic strategy guides and data lists at the start, and got more thorough as games went along. At the time of FFI, Tokuma Shoten/Famimaga were publishing the books and the the 63-page Final Fantasy Kanzen Kōryakuhon was the single book released with it. By the time the FFI/FFII double set were rereleased in Japan together, NTT Publishing had taken over and they published a book covering the two games with roughly the same number of pages dedicated to FFI.
Not in Ultimania Archive, it covers the game as it was, though it does make reference to how the unused Warrior of Light art became it's Dissidis rep. lots of big art pieces for the FFI section though.