raspberry 777
Thank you if I don't know what

I was able to remove the button according to your directions and photos in 10 seconds. For me, nothing is damaged / broken in the button itself, so the problem is probably in the adjustment of the screws responsible for the pressure of the buttons.
I post pictures of my button. For me, they look a bit different because the pushers are mounted on permanently connected plastic triangular plastics. Maybe this will come in handy for how to fix your button.
Or maybe post an advertisement with a photo on the Electrode, Gumtree or OLX in the "I'll buy" option. Maybe someone is good at disassembling or from stocks from a few years ago.
By the way, during the next renovation or construction, I will buy one as a spare (it is not a property, and then peace of mind :)
Best regards and thank you again for sharing your knowledge and experiences. If I can help somehow, write it.
aaanteka
Thank you for the information that this is an old, market model, not available for purchase. It helped me not to disassemble the panel by force, without knowing where the tabs are. If I broke something, I wouldn't have anywhere to buy it :)
Added after 4 [minutes]: As for disassembly, you actually need to move the panel with buttons down - gently and without pulling it towards you. Rather, even pressing lightly against the tiles / walls. It comes out without any problem. Then, when it comes out of the buttons, slightly tilt in the direction in which it bends.