UAPremExecflyer

Welcome to FT!
That's quite a rant for a first post.
I understand you're somewhat frustrated, despite the fact you apparently got a very good price.
But I don't believe that this experience could be remotely categorized as the "worst" car rental experience.
Firstly, you rented a Mercedes A-class or similar. You are not guaranteed a particular vehicle, unless you book a specialty car. The location didn't have the A-class so offered you a similar or an upgrade. No cheating there.
Second, you wanted to take the car into Eastern Europe. If you had done a little research beforehand, you would have found you have to pay extra for this privilege. It sounds like the agent should have been more explicit with you. Still, no cheating.
Third, you admit the car was dirty. Application of this 'cleaning fee' is probably more than a little subjective depending on the return agent. Not sure I see this as cheating either.
Fourth, on adding your FFP to the rental, I'm not sure what happened there. It seems you were already aggravated on pick-up and return that you and perhaps that communication didn't go so well.
All-in-all, you seem to have had somewhat unrealistic expectations, you weren't as prepared as you should have been and to avoid being "sick" in future, I'd suggest you do a little more research before your next rental.