You've had a good diversity of opinion on the questions raised, but I wanted to add one more thought - Talk to your agent.
All of these questions could be talked about with your agent, and get his particular take on them as well. And there is no problem about raising questions and discussions with your agent. I think sometimes when we are new clients, we worry about bothering the agent or looking silly. But I think we need to remember that they know we are new clients and unpublished and they should be very willing to answer our questions.
Don't be a nuisance - ie, calling or emailing every otehr day with what if questions - but there is no problem with occasionally sending emails with questions or scheduling a time to talk by phone to talk through some questions.
Personally, I've heard pros and cons for multi book deals. Mostly dealing with risk assessment - ie, what if you first book's a big hit, wouldn't it be better to shop your next book to the highest bidder, versus what if your first book flops, will anyone be interested in you second or would it be better to have it locked in? And what if the very cool editor you sold to leaves the publisher but you're locked in for three books... etc.
And all the discussion is intersting, but I also think you should discuss these issues with your agent.
If your agent is experienced and reputable, he'll have his own experiences and thoughts to draw on. So, ask him when he'd like to see sample chapters from your work in progress. Ask him about the possibilities, and pros and cons, of multi book deals.
~suki