It's literally just a naming gimmick to avoid having to use numbers while maintaining a logical progression - early in the planning process it was hoped that the campaign would be more "open" and GMs could run those four in any order, or as stand-alone adventures. The ending campaign book has changed since I withdrew my drafts, some details here.
I mentioned it on other forums, but that wrap up is frigging awesome. I'm a fan of IEs though, and my players (after initially betraying her) have taken a shine to Frosty. One player even offered to act as her Charles Foster Ofdensen as the events of DotA unfold. With a good old fashioned immortal elf throw-down on the table that makes my job as GM that much easier. And the players get a chance to be the end of one of the long-lived pointy-eared meddlers I have so much fun tossing in my games.
The bit with the sea dragon sounds epic, and the druids in the London sewers are a great faction out of left field. John Dee is also creepy as hell, and I can't wait to introduce my players to him. The plot is linear, but that's rarely a problem when running a game. I tend to like a linear plot, because my players are always going to take some weird turn and it all balances out. Loose plots tend to be a mess to begin with, and then I have to sit down and work out a plan before I can sit down at the table and rework it in play. It's like doing the work twice.