Right. I would like the world book to be a canonical "Stonehenge 1000" sourcebook for Ars Magica Fifth Edition if possible, but it might not be possible.
Computer games and table top games are different media, and there are different problems for each. Here's a concrete example that's already come up.
In the computer game, Longevity Rituals will not work for unGifted characters. If they did, then virtually every character who was alive at the beginning of the game would have to be taken into account in every storyline, right to the end of the century. There would be no guarantee that anyone would die. No problem for a tabletop game, but for the computer game it creates a huge amount of extra work, much of which will never be used, because most unGifted characters won't be given a Longevity Ritual.
So, it will become part of Ars Magica canon that Longevity Rituals for the unGifted were a 12th (or maybe late 11th, in Thebes) century Hermetic breakthrough. I'm pretty sure we can actually do this; I don't think we've said anything that contradicts it. However, similar things might come up that can't be accommodated that easily, and that might mean that the setting for the computer game can't be, quite, the canonical setting for the tabletop game. We'll be working to keep things as close together as possible, but at this point we can't make firm commitments that we will be able to keep the computer game entirely within tabletop canon. Fingers crossed...
Of course, even if it is, it's set 220 years earlier, so it's still an alternative setting of sorts.
The Diedne book is different. First, a fair bit of it will be for 1220, and thus will not affect the computer game. Second, the magical secrets of the Diedne will be properly integrated into Ars Magica canon, because failing to do so would be too much of a break from the tabletop game. (Yes, of course they will have magical secrets.) I don't plan to describe the plans of the Diedne leadership, or take sides in the Schism War, in the book. So, we can promise that this book will be canonical.
This sums up my impression when I talked to Larry as well. I'm really looking forward to seeing how this develops. (Although I may need to find a Windows laptop from somewhere.)