Wow, some good questions! I don't want to be a big spoiler person, so I'll try to address as many as I can without going overboard...
OK, in no particular order:
-Yes, there will be a standard edition, so far as I know.
-I wrote about ten pages of material of the fictional Space Beast, telling the story of what happened on Fury 161 in Morse's voice. So, like 10 pages from a book. I haven't seen the very final version, so some of it may have been cut... BTW, WAY fun to write.
-Yes, there's a timeline. I did my best, but it was a major undertaking. Thankfully, my editor and his contact at Fox had several uber-fans contracted to check for continuity, so it should be fairly comprehensive. I was asked to reference some of the new stuff coming out (Fire and Stone, the game, etc), so that upcoming projects would be included in canon, but there aren't a lot of details in the report, just references.
-The USCM tech manual was a great resource, but I don't think this book is much like it... totally different visuals, and a lot of Company voice in this one. This is more about WY's take on the xenomorph than a tech book, though there is a lot of tech stuff included. The art is amazing.
-The big mysteries, like how did the egg get on the Sulaco... I addressed some of these things but didn't definitively answer them. Remember, these were movies, there was a ton of artistic license taken when they were made, and while I did my best to supply plausible theories, I didn't necessarily solve anything.
-The WY report is written a few months after the events in Resurrection.
-With Prometheus, I had to be very, very careful about discussing Alien origin, because there are new things being written and I wasn't invited to speculate. That's all I can say, I believe. Except that I'm pretty certain it's Milburn, one L. Online sources disagree in either direction, but it was Milburn in the script...
-The Walmart thing... my editor and I decided that was a rumor. WY just had to go underground for awhile.
-Oh, and I came up with XX121. Honestly, I just put it in as a reminder to go back and put in whatever WY really called the things, but the higher ups decided that XX121 worked just fine. Apparently, that is now the creature's designation. Go figure!
Did I get everything so far? Thank you so much for asking, and for your kind words. I'll still be around for a few days, so please ask more... If I can answer, I will. Cheers!