But as I understand it, those Amex cards are actually Payback cards, with their own separate Payback number on the back, and the points earned are sent directly to Payback (no conversion involved, I think). From there, you have to action the conversion to M&M yourself.

So, with these cards you don't earn any MR points (you're excluded from Membership Rewards because of the Payback connection) or cashback as such. But you are entitled to all the special offers, deals with shops and reductions that you can find somewhere on your profile page (a recent one offered 50% off purchases from selecta snack and drink machines in Germany).