I know this is an old post, but I stumbled upon it searching for something else, and I thought I'd answer your question.
All those keywords being lumped on at the beginning are your categories. By default, zen cart includes your category names in your keyword listing. As per another thread, here are Dr. Byte's instructions for removing them:
a) copy /includes/modules/meta_tags.php to /includes/modules/YOURTEMPLATE
b) find the line that says this:
Code:
define('KEYWORDS', str_replace('"','',zen_clean_html($keywords_string_metatags) . CUSTOM_KEYWORDS));
c) add a line ABOVE that line, which says this:
Code:
$keywords_string_metatags = "";
d) save and upload the file