I have recent experience with a very similar situation. We decided to go ahead and change the entire look of the site and a lot of the content, but we kept the same URL structure. For one of the main keywords we experienced the following:
Google: We fell from first page to fourth page over a period of 4 months. We are just now starting to see an uptick in rankings and have climbed back to the 3rd page.
Bing: We climbed from the third page to the bottom of the 1st page over a period of 3 months.
Part of the redesign was to focus our on page optimization results.
In answer to your 4 questions:
1. Redirect anything that won't be there for sure. You don't want people clicking an old link just to find a 404 dead site.
2. Yes you can redirect more than one old url to a single new page, but make sure it is a logical redirect and is helpful to the user. Don't just do it for fun.
3. Like Joram said, "You'll have to wait and see"
4. Don't redesign just because you are tired of the look. Too many web site owners look at their website everyday and get tired of the look when it is a perfectly fine site. Have a purpose for redesigning that includes something along the lines of, "This redesign will help my visitors because...."