I'm not suggesting using the html catalog at all...I'm suggesting using the full php version of the cart with a Dynamic URL add-on. All of the points addressed above can fairly easily be accomplished with X-Cart - I really think you are creating 10x more work than necessary. To address each specific point:
1. This can be done with this module:
http://phpsitesolutions.com/x-cart_variant_chart.htm
2. You can easily create different layouts for different categories or even different products using {if} statements.
3. If you are on an X-Cart compatible host such as EWD Hosting or Hands On Web Hosting, load times are not a problem.
4. You can control the listing order within a category to whatever you want using the POS values.
5. See #1 above.
6. If you use one of these modules, there is no difference between an html page and a php page:
http://forum.x-cart.com/showthread.php?t=40783
Creating html pages for everything is a TON of extra work, and each time you change something (including quantities, a minor layout tweak etc) you end up having to make the change on ALL the pages. Using X-Cart's default skins, you change it once and you are done. Creating new pages is as simple as using the WYSIWYG editor to add your content.