I can imagine how frustrating this is and would be spitting feathers myself. However my advice would be sleep on it before making any rash dissensions, then if you still feel you want to leave the scheme that’s fine.
I would be on the phone to mb first thing in the morning and tell them you want a like for like car of similar size and automatic, asking them to arrange for a car made available from another Europcar branch, telling them you chose the scheme to have a reliable car and to be always mobile. You could also make them aware its exceptional circumstances to see if they can rent a car from another rental company until one becomes available with Europcar. You could also ring the dealership asking if they something suitable including a demonstrator until one come available.
If/when you proceed with a rental car my advice before you drive it is to take a photo showing the mileage and fuel guage and the same when you hand it back showing a full tank. I also take photos of any damage when I receive a Europcar and before it goes back.
Incidentally what car do you have?