Is there a way to make the user aware that the child link contains sub child links (tabs)?
The current menu does not mark any parent links as having children, but I could add that.
Would I use the skin object twice, once in the horizontal menu pane and once in the left pane?
Yes, you can put as many instances as you want on a page. This site actually uses 5 different instances http://www.littletonpublicschools.net/. One for the Home & Calendars links, One for the Left Horizontal menu at the top, One for the Right Horizontal Menu at the top, One for the Left Vertical Menu, and one for the Right vertical menu when there are 3rd and higher levels.
Will the CSS Selected id work in such a way that if a parent tab is selected it will remain colored while a child tab is selected and that tab remained colored also?
Yes, each child in the selected chain has a SelectedTab selector class added to it, and they can also be styled separately because they also have a selector class to indicate their level.