1) That is on the webassist/plugins/catalog/details.php page
2) The default theme is the main theme, any of the other themes can be deleted, but the default theme is used as the default when the editir is opened without first selecting a theme.
3) On the products update page, you can set the categories the product belongs to, if you want to show the product when the main category is selected, you need to select the main category as well as the sub category
4) I have created a support ticket so we can look into this issue further.
To view and edit your support ticket, please log into your support history:
If anyone else is experiencing this same issue, please append to this thread.