I think you're overthinking things. :)

Main_page is the content that follows the url parameter main_page=

So, for /index.php?main_page=page_2, main_page field is page_2. For the associated_db_id, there is no "unique" value that is associated so it should remain blank. This value is used to target a specific type of "item" for which the URI rewriter is to apply "conversion" a products_id, a category, an ez-page, or a manufacturer. These "items" all have a unique id and that is the associated_db_id.

So depending on your condition/desire, the above example could be entered as:
Uri=/page-2, language=1, current_uri=1, and main_page=page_2