To do this automatically you will need to write an external application, which is possible (I've done several such tools in the past), but more complex than writing a script for Acrobat.
If you want to do this in a batch process, you just use a loop to go over all the pages of the document, and then the extractPages command to save them to the desired path. You don't need to use saveAs, actually.
From what you describe it seems that you can split the file-name using the underscore and then use the first part as the folder name.
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