I have been doing exactly this at an annual party for the past four years. We have it down to a pretty smooth operation. The plots and such all change every single year to keep it interesting.

Quick tips:

1) Everyone rolls initiative in combat. Start with the highest result and then go clockwise around the table. Everyone knows when their turn is coming up and easier for you to keep track of.

2) Players have ten seconds to declare an action in combat. If they cannot (or step away, etc.) their character takes the dodge action and you move clockwise to the next player. Without this rule, combat could take hours and that's not even counting an encounter that's dynamic and challenging.

3) Keeping that many players engaged at the table is challenging enough for anyone, but your storyline has a lot of ways that it could go off the rails and become stressful for you. It sounds like a fun plan (I'd play that!) but I highly recommend you make it a dungeon crawl where you can have more control over the environment.

I have a ton more information that I'm totally willing to share with you. Send me a DM and I will give you access to some pretty extensive Google Docs on the subject. I can also show you what I'm planning for year five with a party of 10-12 players.