It now seems likely (can't be 100% sure due to a lack of responsiveness from anyone at Accor) the the lack of last day breakfasts issue is a web site error.

I made a bed & breakfast booking over the phone (because the web site wouldn't accept Amex for the property I was booking) and the confirmation email correctly showed free breakfast every day, whereas the Accor web site had been displaying free breakfast for all but the last day, before I gave up on account of the Amex problem.