I'm not privy to a lot of ins and outs of SEO
with database applications, but I may be able to speak to part of this. You're trying to use one page to simplify maintenance. So you're nervous about having a number of different pages in various folders, is this correct?
IF I'm understanding the problem correctly, there are a couple things I know of that could help. One, you know how when you create a new webform, it lists the name of the partial class (usually the same as the filename you created). Swap the default out for a base page class object. One that will let you reuse the same .cs file for every page that you're building. That would be one thing to help maintain consistency across any number of pages AND be able to change the behavior on all pages by updating one code behind.
Two, check out master pages. I admit, I'm only just starting to work with them myself, but the idea is very attractive. It's essentially SSI that's compiled into the page so that it doesn't require the server hit SSI takes. It lets you design an interface for any number of pages, implement it as a user control, and change ALL pages by making changes to the one master page. If you use CSS, which granted .NET does not make easy, you can control the page interface from the master page, the programming logic, from the base page code behind, and the appearance from the CSS stylesheet. That's pretty cool.
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