Yeah that video looks helpful, although you may not necessarily need to remove all the rubber pads, just identify the malfunctioning note and zoom in to the rubber pad that corresponds to that note. One rubber pad for one note, u just need to manually count your way to the correct one. Remember to work in a clean environment and switch off any fan or aircon - you certainly don't want dust blowing around and getting into the rubber contacts.
Medeli keys? that might be a real problem then, most probably an inferior quality keybed compared to Fatar. To be sure, I suggest you open up and clean the contacts yourself using a q-tip. There's every reason to doubt that the service center actually did it unless you saw them doing it in front of you. If the problem lies in the key-contact PCB, then you would only need to replace the key-contact PCB and not the synth motherboard. A typical key-contact PCB only costs $75 USD - I imagine that the $900 they want to charge you is for the synth motherboard, which in this case would NOT help if the problem really lay in the key-contact PCB.