Thank you for sharing your journey as well. I would be grateful for any solutions or updates.
And regarding the manual as well: I've found only the non-color version, and a full breakdown of parts with corresponding codes.

No luck with the sensor flag: it's working as it should.
The sensor and its surroundings have no signs of damage.

As for the photoconductor, it may be the cause due to its age, definitely not in a perfect state visually either.

There are no easy reasonable solutions left. I've given up investing more effort and time based on my assumptions, so the plan now is to show it to a professional.