Piero Custom Bone Shapes & Rig Mirroring: Duerer's Progress

Do I see it right that a correctly mirrored bone does have the same "Roll Value" as its counterpart from the other side, just with the opposite sign (for example 5° => - 5°)?

"Bone Rolls" and "Arm IK Pole Angle" of the right wing corrected:

Some drivers on Piero's right side had to be corrected:

Not yet 100% working but getting closer.

My finished rig here:

There seems to be a broken "Driver", but I can't find it in the rig:

duerer remove the word var from the Expression field and type in var and the Error is resolved:

I have absolutely no idea why that works.

Thanks, spikeyxxx! The error message disappears but reappears as soon as I click on "Update Denpendencies".  Furthermore, the red line under "pose.bones["CTRL_armSpline.001"].constraints["Copy Rotation.001"].influence" in the "Drivers List" on the left side of the image above stays. It refers to a bone "CTRL_armSpline.001" which no longer exists (as far as I can see) because it's one of the bones which are either on the left or the right side of the rig and therefore have been renamed with a suffix ".L" or ".R" using Blender's renaming functions.

Right, that is way too complicated for me :) Too bad it didn't work.

If I turn on the visibility for all bones and bone layers and search for the pattern "CTRL_armSpline.001", I only get "CTRL_armSpline.001.L" and "CTRL_armSpline.001.R". It seems that somehow this "Driver" data block marked with the red line in your screenshot above "survived" the renaming and is now useless.  "CTRL_armSpline.001.L" and "CTRL_armSpline.001.R" by the way don't have any constraints or drivers. I'll delete this broken driver and test the rig afterwards.