Very Detailed Post Frosty.
Thats great
Is the issue the site stucture or the number of sitemaps?
Have you tried one sitemap at
http://www.yoursite.com/sitemap.xml
this would be beter from an seo aspect because your a,b,c, pages are closer to the root url.
Normally most of the links to a site go to the root url is
http://www.yoursite.com.
This normally means that the root url is considered to be the most important by the search engine and the closer your other pages are to this page then the better they will rank in the search engines.
It is all to do with linking and how the PR (page rank) is ditributed through the site via those links.
So depending on which pages you want to appear high in the search engines depends on how you do your linking.
Also by distributing the PR around the site it has a multiplier effect which means you can get more PR by linking the pages in a certain way than the sum of the PR of all the pages.
So I would be interested to know if you tried
http://www.yoursite.com/sitemap.xml
and no other sitemaps.
Thanks for all your info and sharing. I didnt know some of those sites you mentioned.
Glenn