I do understand your frustration, room and especially bathroom looked horrible.
It's not a 4 star for sure but it shouldn't be a 2 or even1 star either. I refuse to accept an argument that in Europe hotel star rating is different, place looked like a health hazard. I'm very sorry for your ruined vacation.
I have never used lastminute.com, but many times i used priceline and hotwire and i have never been cheated, even for international hotel stays. Obviously it does work with some travel sites and it doesn't with others. Thank you for your warning! I wish you that lastminute.com wil resolve it quickly.