One really BIG ooooops is you taking your own parts. I know you think it saved you money, but they probably charged you double on labor...
When you go to the dealer, don't take any parts or oil with you. IF you want to do the oil changes yourself, specify that don't do it during their check over.
As far as the quality, it sounds as if a trainee was doing the work.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS