As far as IP using the bias for dark scaling – I believe the methodology is to subtract the bias from the dark so that only the dark signal is left. Then the dark can be scaled. So, let’s say I have a 5 minute dark and want to use it for a 10 minute light. If I subtract the bias from the dark, I can scale the dark x 2 to match the (presumably linear) dark signal in the 10 minute light. Then I can separately subtract the bias from the light.
Personally, I’m wish you – I avoid these “short cuts” and much prefer to use properly matched calibration frames.