Ikonta 522/24 (Contina I) repair

I've had this Ikonta / Contina I for 3-4 years. Bought as-is with stuck shutter for maybe 10€, thinking "I'll manage to fix it". So now its turn has come.
Someone had been there before: damaged screw heads and the like. Straightened a bent lever in the Prontor-S. Cleaned the retard mechanism, and (against the advice of Messrs Gauthier) lubed it with Moebius oil. Now all speeds work and sound OK by ear; will measure later. I bet the camera can take pictures now.

I'm still stuck on two points, and hope someone might provide information.

1. The aperture scale is offset, i.e. when dialing f/4 it is more like f/8, etc... I might make a new scale but I'd rather find out what went wrong. For this I need to separate the shutter assembly from the front plate. There is a nut on the inner side, but I can turn that by maybe 10° before it is blocked. Afraid to apply more force in case there is a set screw to avoid rotation of that nut.
   Is there a trick?
2. There is something in the viewfinder: a debris of some kind, or a bug? Right in the middle. Also some dust and haze. Tried to remove the top plate. Film type indicator: easy. On the other side, a screw that just uncovers what seems to be a tripod socket on the top side (???) but the base of that tripod socket(?) does not want to move. I even drilled two small holes to use a lens spanner: no way, moves just a little. On the underside of that, at the top of the film cassette chamber, a metal disk with two small slots at the outer edge, that might be suitable for a spanner if it were not facing the rewind fork less than 2 inches away.
   What is the method to remove the camera's top cover?

Thank you for reading.