While doing some research for an answer, I discovered the Redwall series book Marlfox. At the end of the book, there is a meta-note:
This narrative has been edited by Florian Dugglewoof Wilffachop, Actor Manager Impresario. Who insists that the entire tale is a drama, which he will be later performing as a play, hence the three parts being named as acts rather than books. We crave your indulgence for this deviation.
So... did anything in the book actually occur? I'm inclined to believe that it did, and only small details were changed - otherwise why would this be a book released as part of the series? The "play" bit would then just be a framing device to tell a true story. However, not having access to the book (or many other Redwall novels) I can't tell for sure.
Is there any textual evidence, from other books (assuming that perhaps Marlfox can't be trusted), that the events in Marlfox occurred? Do any other Redwall books refer to events (preferably multiple events) that happened in Marlfox as fact?