There's a lot in your question and there are threads that address parts of it. So let's start with those parts, but first a question: What is a mobile site for you? It could be anything from a fluid width design that works well at small resolutions, to a much, much lower bandwidth version of your site that has no images, very little CSS and so forth.
The Drupal part - you could create a theme that would work well at different resolutions. But even if you use display:none for some page elements, you're still sending them and using the bandwidth.
The static HTML part - that depends. If it's mostly CSS-based, you might be able to create a layout that works.
Duplicate content and SEO - there have been lots of discussions on this, some going way back
...] (dupe content, 2006)
...] (fluid or fixed for mobile, 2012)
...] (subdomain or new domain for mobile, 2011)
...] (dupe content, 2008)
...] (SEO differences, 2012)