Hey John,
1. I just added the embed code provided by YouTube into the detailed description of the product.
2. (a) The horizontal menu is javascript and not populated by x-cart itself. It's a manual update made by me when we add or change categories.
(b) I created a separate file called header_nav.tpl from the existing header table. I just modified it to be 2 cells and moved the logo to the right side and kept it's existing HeadLogo class.
3. You modify that area in skin1/customer/main/welcome.tpl
You can just use regular html to add images or text. I put that particular image inside a div.
4. Width can be modified in skin1/rectangle_top.tpl
Mine is set to 1000px.
Check out this thread:
http://forum.x-cart.com/showthread.php?t=21529
It has some general info on what some of the more common pages do in x-cart. It's a good starting place to get a better idea of where to look for different things.
Good luck!
Brad