You can use the same m101 field with same exposure times 120s and see if the flaring is detectable in a single frame. There are a few open clusters in Auriga (fairly well placed now) that might work. Then try the fine focus test.
After plate solving an idk10 image and using reported image scale, the focal length comes out to 1672mm.
That's computed the focal length given plate solved image scale and known pixel size.
Which plate solver did you use that reported focal length? I'd like to give that a try.
I don't know if using a Ronchi eyepiece can be a reliable method to tune the primary to secondary distance. We know AG optical instructs to only adjust 2 of the 3 collimation screws on the primary in order to not change the P to S distance tuned at the factory. Maybe this distance is very slightly off.
The flares are detectable in a single frame. I have 59 of them I can upload if anyone is interested in looking at them. Auriga is in a tree for me, so unless its in the north, high east, or extremely high in the sky toward zenith, it will be off-limits to me.
All of the folks I have asked about plate solved FL report very close to the spec for their scope. The spec is 1674, so you are well within that and you are not seeing the flaring problem I am. I used PixInsight's ImageSolver script to solve the image.
Let me ask you another question, since you own a 10" can you see the center white ring and spider set in the below image? The very inner one, not the larger two. Even with the secondary baffle removed on my scope, I cannot see that.
Lastly, my version of the iDK does not have a black screw on either mirror, and the version of the manual I have does not talk about leaving screws alone. The older ones do, but my version does not.