I received more clarifications from Accor:
- About the access to Diamond benefits while booking a ALL PLUS rate: "the discounted rate with which you booked [ALL PLUS ] cannot be combined with the usual membership benefits you could have received as a Diamond member. For those to be applied the reservation needs to be booked at a standard loyalty rate"
- About the inability to book a non ALL PLUS rate when logging in with an Accor account: "Keep in mind that in case some rates are not displayed on the website or on the app, you can always choose to make the reservation through our reservation team office by phone or by contacting the hotel directly."
- About the T&C entry that rules that Diamond benefits do not apply wiht ALL PLUS rates: "On the T&C of the ALL PLUS Voyager subscription it is stated that the discount offered by it cannot be combined with other permanent discounts, and the complimentary breakfast benefit it is de facto a permanent discount for Diamond members in our hotels."
- About some properties granting Diamond benefits over ALL PLUS bookings: "Accor doesn't prohibit its hotels to apply the loyalty benefits regardless of the rate chosen, especially to Diamond members. Some properties are more strict than others with regards of the application of the terms of service"

Conclusion: it's a total mess, nothing's predictable