OK, some good news concerning my spinners.
I have brought my panel to an electronics shop nearby, and the owner was able to detect some broken tracks on my P1 spinner board. He fixed them and this spinner now works almost perfectly. Sometimes (rarely) the spinner just does not react when turned. The thing is, those spinners seem to need to be very precisely set. The spinner board must be set at a very specific position to detect the encoding wheel correctly, and it might still need to be adjusted a bit.
The P2 spinner does not work yet. The tracks seem to be OK, but it does not work when plugged as P1 either. For now I don't know if the opto switch is dead or if it's a matter of positioning, but so far it as given no signs of life.
It was late when I got to the shop and the owner was a bit lazy to plug on the oscilloscope, but we manage to ascertain that my pinout was indeed correct. I'll post a picture with the full pinout when I have time.
By the way, I'm playing Pop'n Bounce on MVS, and the game has a Paddle control mode in the settings, and I was wondering how it was supposed to work and could not find any answer. Was there some kind of IO board like Irritating Maze? Does anyone know anything about that? That game is tough as nails by the way lol.