1 - The MYTHIC MAP PACK will NOT be released in conjunction with Halo 3: Recon (Fall 2009). They will be bundled with Halo 3: Recon; but there will be new maps altogether that come with H3R as well. By then all 6 maps in the Mythic Map Pack will have been released months earlier.
2 - The Mythic Map Pack WILL have 6 maps. Bungie hasn't confirmed a release date, but I'm assuming it will be released sometime in February. Here's why: The LIMITED EDITION of HALO WARS will include a code to download 3 of the 6 Mythic Maps... so buy the LE of Halo Wars and you get half of the Mythic Maps before everyone else. The ENTIRE map pack, meaning all 6 maps will most likely be released shortly after that.
3 - You don't have to purchase the Limited Edition of Halo Wars to purchase the Mythic Map Pack, 3 of the Mythic Maps, etc. All of the Mythic Maps (meaning all 6 of them) will most likely be released shortly after the release of Halo Wars LE. Again, as I said before, if you PURCHASE Halo Wars LE, you'll get a code to download 3 of the Mythic Maps (Assembly, Orbital, and Sandbox).