Thank you for the answer, even though it wasn't the one I was hoping for.

True, it's impossible to prove that what I booked was the cheapest option, but wouldn't that be the case even for a regular (cash) ticket? The cheapest available now is a semi-flex economy fare. How could I prove that there wasn't a cheaper non-refundable fare available?

I wonder if I could get some kind of a "quote" emailed to me by the airline, confirming that this is the cheapest fare now available? Maybe that would carry more weight in court than a screenshot, as it could - at least in theory - be verified with the airline?

Re the business class: I'd be OK with them reimbursing me for the number of miles required for an economy-only award (which can easily be looked up online) - but it does complicate matters, as you say.