I dont know about your dealer but Calabasas Motorcars is one of the best dealers I have ever been to for service. Other service advisors wont even talk to you if you havent bought a new car in the past month but I have seen advisors in shirt and tie getting dirty trying to fix a 190E. I dont know about your dealership but I suspect if they are tying to falsely compound your problems you should contact MBUSA.
$80 an hour is cheap if they fix the problem with the right tools, right equipment and procedure the first time. I wasnt aware of your H20 pump problem but I suspect some other components were in need of repair for the bill to become $5300
Just imagine my world in which I pay $130 per half hour to temporarily fix a problem and then have it give me problems in a week. I think MB technicians do have the habit to replace any component that shows slightest wear because they are accustomed to having MBUSA picking up the bill.