1. You can make your site bigger than 1024 px. It's just not good web design practice. You can make it however big you want.
2. Can't remember off the top of my head. I believe there were a couple of "align" tags that were misspelled.
3. You may need them, but if the numbers do not add up, the browser is going to add more columns so that they do. The result is that it will push out your site further than what you may intend for it.
4. No, there was an empty colspan. It opened and closed with nothing in it. It may have been auto-closed by whatever program you were using to create your page.

Further, another thing to note is that while all of your tables are 1200px in size, you may have forgotten that the main page is composed of several templates, not just the index.tpl (there is also the right.tpl, header.tpl, and footer.tpl). When I looked at it the other day, I noticed that you had tables running all the way up to the right.tpl tags. You may want to check that as well.