I like how Amazon's CloudFront does this. It uses a CNAME to route it to the closest download site. I would do the same. Use something like Round Robin to split your downloads up. Or use a cloud based host to do it for you.

As far as SEO goes, it shouldn't matter. Google will do its own DNS and caching.

Those are some sloppy URLs. I especially advise people to avoid the problems of relative paths in ANY URL. And, yes, <base> probably isn't helping.

Links starting with / are fine. That's the root of your site. Anything using "../" should be nixed and use a fixed path. And never, ever use "./".

I wouldn't just drop the old site. it has SEO momentum and you want to capture as much of that momentum as you can before you drop it (otherwise you need to build it from scratch on your new site).

There's going to be penalties in doing it either way. You're going to have duplicate penalty until the old site gets de-indexed.

Wordpress.com is a whole different beast from the Wordpress software. WP.com uses the WP software and shoves it into a shared hosting environment. So you can't do most things you can do elsewhere.

If you are using your own domain, just move your blog off WP.com and host it yourself. You can retain the same URL structure doing this.

If you're using myblog.wordpress.com, you're a LOT more limited. My suggestion would be to do a poor man's 301. Copy your content to the new blog, then gut the old URL and put a link to the new URL. This is not the preferred method but it lets you keep your traffic and still pass some SEO. Since it's not duplicate, it will eventually cause your new page to rise and the old to fade.

You could go rip the entire site to your local drive using a program like Black Widow. Then you could use something like Notepad++ to search the entire directory.