I'm back bringing some news about the issue.
After trying to take the keys off using thin plastic cards, I decided to buy a pair of snap ring pliers as the Kawai ES4 service manual recommends to use to take the keys off. They worked flawlessly, for this and cost me around ~8 usd.
After taking the ~28 or so screws out and taking my higher D key out (D5?), this happened:
They key came off with the action attached to it, we can see below that this thin felt was completely destroyed and the plastic action bit of the key was sticking to it (as seen above):
It seems that this issue cant be avoided and will happen to the most used keys on the piano, we can speculate that Kawai knows that with this kind of design will lead to this problem eventually. I'm not hating on them, I think this action is quite good but maybe it could last longer with just little tweaks.
We can see in the photos below some used but still working as intended hammer action stopper felts (is there a better name for them?):
We can also see there's a white smooth guide tape to prevent that this felt unsticks with use (it guides the key down to the middle of the felt)
So I took the remains of the this stopper felt of, as we can see it is quite thin. I think if this felt was a little bit thicker the action would be quieter and the stopper felt would probably last a little bit longer.
So right about now I didn't know what I could use to replace this thin soft felt. The only thing that came into my mind at the time was to use the little red soft rubber (I don't remember the name of this thing, it's soft and synthetic rubber), I though to myself this isn't going to last long as it is getting hit so many times so I wrapped it with PTFE tape (to give it structural integrity or so I thought) and used some electrical tape (looped on it self) to stick it where the felt was.
I shall call this the 3rd world country temporary fix, I'm going to look around for a better fix (felt and sticky tape) while I'm stuck at home because of the pandemic.
The key works, but the bottoming out of the action feels different as it gives it a bit more resistance, as my temporary solution isn't as soft and thin as the felt it replaced.
I just hope this fix last at least a few weeks/months, and hope this improvised felt gets a little softer with use. I will leave a video below so you guys can hear the difference.https://drive.google.com/file/d/1AQFzveKxB1QHViGjlzat9i4MEngHhzXz/view?usp=sharing
As for the middle C and D keys, I couldn't find anything wrong with them (the stopper felt was quite used up as well tho) so I just took them out and cleaned them a bit before putting them back on.
Thanks for reading, I would love to hear suggestions!