Sweet!
Please, no. Some things are better left mysteries.
Looks like I got my Certamen app, and then some!
I've always been open to the idea of a source book that has three or more mutually incompatible takes on them. The idea of a definite take on them in canon upsets the crotchety old old fart in me, but not any more than the present take on house Tytalus (which I'm not as fond of as the Tytalus in prior editions).
So while some things are perhaps better left mysteries, I'm cautiously optimistic anyway taking the bad with the good.
Does having guessed correctly win me a free copy? 
Doesn't matter, I'd support the kickstarter anyway. I'm very poor, but Ars is my favorite place to put what discretionary cash I do have.
No, you were too slow. 
The video game starts in 1000 AD, before the Schism War, so you can play the Diedne. A sourcebook rather becomes necessary. We need full details of Diedne magic, for example. On the other hand, you don't get to play the leadership of the Schism War, so we won't be defining everything about who was right and who was wrong, as the description for the Kickstarter says.
The sourcebook will include a section on how to use the Diedne in 1220, and that will be options: Legends of Hermes/Ancient Magic type stuff, a couple of return of the Diedne options, maybe something else. Unlike every other case, this book has, of necessity, been announced before we've really started working on it, so the contents could change, but those are the current plans.
And the World Book? Is that more of a computer game guide, like the Players Guide for Skyrim or Dishonoured, or is it a tabletop game book?
Will this book be available outside the kickstarter project in any format (hard cover, soft cover, .pdf)?
Same question for the world book.
We don't know yet. I'd like it to be a tabletop game book: Stonehenge 1000 for ArM5. That will depend a bit on how close to tabletop canon the computer game can be kept. The world book is a bit open for development yet, depending on what seems most useful given the way the project goes. It is, however, likely to be useful for both the computer game and tabletop game.
If the Kickstarter doesn't fund, neither book will be available at all.
The Collector's and hardcover editions will definitely not be available outside the Kickstarter project. Softcover reprints at some point in the future are a possibility, as are PDF sales, but not for some time after the computer game releases, and possibly not at all; that's something we haven't decided on yet. After all, the Kickstarter hasn't funded yet.
I'd love to help fund it, but I really don't see myself being able to afford (the rather steep) $150 before the end of November to get my hands on the hardcover Diedne sourcebook.
It's important to remember that this Kickstarter is not for the Diedne book; it's for the video game. Printing the Diedne book costs money, and writing it will cost some money over and above the cost of making the video game. That means that, in the planning, you have to make sure that you can afford to pay to develop the video game if every single supporter chooses the lowest level with the Diedne book on it. $150 is apparently it; I haven't run the numbers (not my job), but it sounds plausible.
We were thinking about doing softcover releases of the Diedne and World books at some point after the computer game comes out. (Obviously, since the Kickstarter hasn't even funded yet, those sorts of plans aren't at all concrete.) It would depend on demand. Pledging at $20+ and saying that you want a softcover Diedne book would increase the likelihood of that happening. If a thousand people did that, and the Kickstarter funded, then we'd definitely do a softcover edition.
If the videogame kickstarter does not manage to reach its funding goal (and despite wishing Black Chicken all the luck, from what I see I fear it will not even reach the $100K mark), is there a chance Atlas will independently develop the Diedne stuff in the next year or two?
No. The next couple of years of Ars are already mapped out, and except for the chance of it happening through this project there is no Diedne in it.
It's a pity the deadline is so short in that respect.
Is there any chance, say, I could pitch in $20 toward Sub Rosa pledging, so they can use it as a competition prize? I suppose that would mean people would not fund the kickstarter because they might hang out for their chance at a prize, so that would be a bad thing... I'm just not sure what I can do as an author to make the kickstarter more likely to come to fruition.
Although I have to say, I'd like to congratulate whichever of you ponied up for the $2 500 level. That's, well, basically insane and I commend you for your sheer nutjobbery.
Does Kickstarter even work outside UK/US(only ones mentioned by the site)?
Either way, i would be 99% likely to buy a Diedne book and quite interested in the Worldbook, regardless of anything else (I can certainly not pledge 150$ unfortunately).
It does. When I go there it gives me adds for projects in scandinavia, and I succesfully supported the OotS drive a while back, despite not being from either the UK or the US.
I went through a similar thought process. How much could Sub Rosa pitch? Not a huge amount, if I'm honest. And taking another tack, would a free issue of Sub Rosa incline people to drop $150 on the project? We're good, but we're not that good. I'll chat to Ben and see if there's anything else we can do. I suspect there is in our own small way.
Maybe the best thing to do is to get the word out there and seek support.