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Neither helps or hinders your SEO efforts but forcing one over the other will help on many fronts (particularly with PR concentration and indexation issues). Generally, I prefer non www URls because this reduces the length of all URLs by 4 characters. A bonus is if your domain and file names are short you may not need a URL shortener on Twitter.

However, there are other considerations for an existing site. For example:

1. what does your link profile look like? Are your most valued backlinks pointing to www vs non www (there is slight PR leakage with 301 redirects)

2. What does your indexation look like? Are more www pages indexed than non and vice versa

3. Do you have a  massive amount of print collateral or ads with www? This is less of a concern because of the 301 but it brings back up issues #1 and 2.

There are other considerations but these are the ones that come to mind quickly.

I agree. It's akin to sending the post office a forwarding address when you haven't moved yet.

Wait until after the change, then update the sitemap. Consider keeping the old URLs in the sitemap along with tne new ones for a bit and make certain you have 301 redirects from the old to the new URLs. You can remove the old URLs once they've dropped the old URLs from the index.

Does one page need to do all the work or can you create 2 pages. When you optimize a single page for multiple keyword phrases the pages tend to become more like SEO gibberish than useful content to the user and it becomes difficult to keep the keyword repetition to a reasonable level. Separating them will also make it easier to write your titles and h1s.

Avoid multiple hyphens (eg key-word-stuffed-subdomain.example.com). Your example only has one-not a concern. The SEs are able to read domains/subdomains with spacers or not. Not a concern there either. In the example you've provided, the issue is more about usability. If you ever have to speak a URL over the phone it'll be much easier without the dash.

This is sound advice. Test out a percentage of pages before rolling out the change site-wide.

I also agree that 18K duplicate titles isn't helping the site.

One thing I would do is review analytics and define the top X % pages and hand optimize those. The balance can be optimized via rules utilizing the system you outlined. As to whether to use the H1 or the file name or some other element, I'd probably lean towards the h1 as it would likely accurately describe the content and not be truncated or contain stop words.

I suppose that we're not understanding your concern. Is the concern over incorporating a keyword or is the question about whether a hyphen has negative consequences?

If it's a keyword issue you can use keyword.example.com or key-word.example.com. No difference from an SEO perspective. It's more about convenience/usability (ease of conveying the address via various media).

I doubt that one dash will cause indexation/ranking issues. I don't see this as an issue.

SEs read .php files so this change probably isn't necessary. If you did, you might establish rewrite rules in .htaccess and add 301 redirects. But again, why mess with the file extensions? Are they having indexation issues?