I've never been a fan of the whole PR-as-primary-SEO method. PR was intended to "get the word out" to as many sources as possible. It's paid off for us in a few instances because a local TV station has picked it up a couple of times and given us a TV story (with an accompanying website article, although not often with links). As far as SEO... it's a lot of work for little reward. Since your PR is likely identical to many sources, it would indeed be pared back in the SERPs where only one would count and the rest would be considered duplicate.

In short, I wouldn't run a mass PR solely for SEO reasons (especially not as a reliable source of links). PRs can get quality backlinks, but only as a bankshot. Instead, I would put the PRs on your sites and see if people will link to it. If it's well written and interesting, you could get some good one-way links. I am willing to bet your competitors are not doing PR for SEO.

I don't know that it much matters. The localization signal from just the ccTLD is not terribly strong but it should be the same for both the older .com.xx as well as the newer .xx

For purely aesthetic reasons I would choose .xx

Couple of notes for you

1. There isn't any SEO impact on WHERE the data is loaded from. Look at any major website (especially one that ranks well) and they're openly using content delivery (like Akamai, Amazon S3/Cloudfront, etc) for static content. This is good business practice because it takes that load off your web server and often places the content closer to where the client is. Faster content delivery can help SEO if you have a slow server.
2. If they're using the raw S3 buckets I would HIGHLY suggest signing up for Cloudfront. There's two benefits to doing this. First, you put the content into Amazon's cloud, where it is more readily available. Second, you can use domain aliasing to help obscure the source. For instance, let's say you have an images bucket. You could add a CNAME DNS record for images.yourdomain.com and then put that into your source code. You can still see where the DNS takes you, but it's not obvious to the general public. The cost difference between raw S3 delivery and Cloudfront is negligible.

Oh, and I use Amazon Cloudfront for my delivery. Never had any SEO issues with doing so.

I use a CDN and have never seen any impact on my SEO. As long as your HTML is served off your domain you're fine.

If you're using AWS Cloudfront you can set up a CNAME so that your content comes from, say, images.mydomain.com instead of xyz.cdn.com. Helps obscure it from visitors and looks a bit neater.

I would pick #2, where you process .html files with PHP. Changing URLs involves taking a temporary SEO hit and I would not recommend doing it.

Your htaccess file can do the 301 (it's actually a config file you can control). Here's some sample code that should do the trick.

RewriteEngine on
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www.domain.com
RewriteRule (.*) http://www.domain.com/$1 [R=301,L]