I haven't been too active in FG lately unfortunately,and I'm still using 3.0. So I don't know how well the planes work on the newer version of FG.
There is a issues with the C170BW file that I remember,(that's why it's in the development section) I'll have to go back and look at that.
I have no idea what the Rembrandt problem is, I always have Rembrandt disabled and never had an issue.(Note I don't really build from scratch so that may be a problem with the base plane or Blender export.)
Try deleting the shadow file, that may be the cause of the grey square.
As for the Pa20 engine noise and gauges that has me puzzled. That's one of the best planes aside from the steering.
The Turbo 170's problem may be due to the engine swap, some factors may not be there for all the instruments and sound.
Glade to see your like the planes
,I'll try to work on your complaints,Also maybe some new plane will come.