There is a further screw hidden under the black cover on the left hand side of the camera (visible in step 1).

Without removing this silver screw, it is not possible to take the camera apart.

There should be an explanation about how to properly remove the ribbon cable. One has to flick up a dark part attached to the ribbon cable socket (opposite to where the ribbon cable enters). Putting the cable back is not “easy”, it is rather fiddly. One needs to put down the dark part again, after inserting the cable.

The three screws for the silver backplate that presses the sensor against the spacers and lens need to be tighten just the right amount. One cannot just fully tighten them. These three screws determine the distance of the sensor to the lens and also its alignment to the lens. One essentially needs to perform a sensor calibration when putting the camera together again.

Finally, the part replaced in this guide is not a “reflective mirror”. It is probably a low pass filter, potentially an IR filter or both.