If the products are all but the same besides color, would you be able to implement something like amazon has, where each product is only one page and you select the color/size option within the the product framework?  This way instead of having a bunch of pages that are basically the same product, all of your effort can go to one page per product.

Have you tried searching the account number to see what other websites (if any) are using that account? If you client's UA number does not end in a -1 then there is a good chance whomever made the site used the same account for all of the work they did, including their own site.

Meta descriptions do not effect your page optimization in the sense that a search engine is effected by it, but it will effect how often people click to your site as the meta description is the copy a person is shown. You should make your descriptions informative and truthful so that it accurately portrays what each page has to offer giving a potential visitor the best ability to judge what content will be on the page (and thereby lowering bounces/re-searches). This being said you should write custom meta descriptions, not just let it truncate content already on your page.  I would start with products that are relevant to the most used keywords and work backwards, if you dedicate yourself to doing at least 20 descriptions a week you will have all of your legacy products done in a year.

Write as much copy as you need to convey the information that is needed for your visitor on that page.  If you can create a good user experience on one page with 100 words (by answering the pain that they would be going to that page to solve) and then you need 1,000 words for your next page, then do that to.  Don't make arbitrary copy limits or goals based on SEO, make your pages user friendly and your copy useful and your pages will rank well for the appropriate content.

The easiest way to do this is to export the data and work with it outside of Google Analytics. I reccomend using http://www.nextanalytics.com/ to automatically export data from GA into excel.

You are correct in your assumption, if you are not allowing Google to index those pages then Google image search will likewise not index images from those pages.  While you could then create gallery pages for your images that did not have noindex meta tags, without having other information on those pages the images will not rank well (and at that point you would be better spending your time just creating unique content for your similar product pages).  Unfortunately, if you want your images to searchable you will need to make your pages as well

What do you mean "pages that have nothing to do with my site" are these not on your domain or are they on your domain but you are not familiar with them?