Just wanted to add my own successful DP of redeeming Accor points at Fairmont Banff Springs on a Virtuoso booking.

I chose Virtuoso instead of Accor STEP because Virtuoso was offering an extra complimentary amenity (free spa access without having to book a treatment) that STEP was not.

I booked via Live Luxe Travel Co, who were responsive and helpful with ensuring that my ALL number was properly attached to the reservation.
As soon as the reservation was made, I was able to see it in my account on the Accor website.

At check-in, I asked to redeem as many points as possible to offset the room charges. This was promptly done without question (though they required a 2FA code sent to the email on file to complete the transaction).
At check-out, I again asked to redeem as many points as possible to cover the incidentals that had been charged to my folio. No problems here either.
So it is definitely possible to redeem Accor points at any time during the stay, even when booked on certain program rates that are not directly through Accor.

I can also confirm that they were able to redeem points in increments of 1,000 = 20 EUR, not 2,000 = 40 EUR. So FT's wiki/FAQ on this is out-of-date.