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Theoretically, if the URL is blocked  by  robots.txt it should not appear in the index results no matter if they are in the sitemap but I have seen URLs indexed that are blocked by robots.txt but are in the sitemap and have good links pointing to it. If you want to block pages that have good links pointing to them, my advice is to remove them from sitemap. #justathought.

About URLs from multiple domains, I personally create separate sitemaps for different subdomains and link to main sitemap and I see better indexing that way.

Again, these are my personal experiences and not rules so please do keep that in mind as things can be different fro them.

As far as my info goes, Google can read ajax as well as iframes maybe not as good as normal content so saying that you are going to add content in iframe so that Google can’t read that, I believe Google will still be able to read the content within the iframe to an extent.

Google consider two pages as duplicate when content on the two pages are completely identical or good amount of content on both pages are exactly similar if this is the case, adding more unique content to product pages can help if only the customer reviews are identical, I personally don’t think its an issue.

Just my 2 cents.