Unfortunately, it’s not particularly unusual for us to have to scrabble around in various of the Fulfillment Reports themselves in order to reconcile everything fully.
There’s been a good deal of speculation about this question - both here in such SAS FBA Category threads as this one:
as well as over in the NSFE & related discussion venues.
Unfortunately, I don’t know the answer to your highly-pertinent 2nd question, primarily due to lack of first-hand knowledge (because it’s been many years since we went out of stock for our Offer-Listings).