Regarding the top horizontal 'tab' menu,
I think things would be a lot easier for you if you had a top-level menu tab simply called 'Store' or 'Our Products' under which all the categories and subcategories reside. This would mean that at any time in the future, you could add categories as required without worrying about the layout of the page.
There are also a large percentage of people (56.15% worldwide, 52.22% USA, 58.90% UK)* whose screen size is 1024px width requiring a max width of around 980(960)px content area to avoid horizontal scrolling.
Having all the main categories showing in the main top-level menu will severely restrict your options if the categories change later on. The sidebar menu could then be used to show the sub-category headings per category/page.
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* aboutus_pressbox43-screen-resolutions.html