Thanks so much for the reply Jörg.
I did end up having success fixing the problem this evening.
As you suggested via email, the problem seems to have been the keyboard bed.
I carefully unscrewed the virus, removed the keyboard, disassembled it and cleaned all parts of it - using alcohol where necessary.
I discovered that the two long green circuit boards on the back of the keyboard (which carry the rubber contacts) where full of black soot particles if unknown origin.
I am assuming that this dirt might have created unwanted electrical contacts and changed the resistance/impedance.
What I used to clean it off was mostly kitchen wipes, sometimes with a little alcohol.
I tried to stay away form the black contacts that are touched by the rubber keys.
I did remove and reattach the rubbers however in order to make sure there were no particles left underneath.
(btw - Re-attaching the rubber elements works with a thin metal piece, like a nail, best with flattened tip).
Summarizing, it seems the problem of uneven velocity response in our case was caused by:
Dirt on the electrical circuit in the keyboard bed.
Not under the rubber switches themselves, but around the soldering contacts on the circuit.
The work I had to do to clean it up was pretty extensive, and of course doing this there is never a guarantee that one doesn't end up doing more harm than good,
so I can probably recommend leaving this kind of repair to an official workshop.
Really glad that it works again now though.