Is there a way to mass-delete everything that I've "archived" (and, ideally, received before X date) while preserving everything that's still in "inbox," regardless of whether it's read/unread?
you may be able to apply a label to everything you have archived and from before a cutoff date, and then open your account through an IMAP client that will parse the label as a folder in its ui w/everything grouped together, and then mass-delete by nuking them through that application.
doing this in the gmail web interface is gonna going through the messages one page of results at a time, clicking select all, clicking delete, and then next page.