With the backlash test failing it's sounding to me like a mechanical issue. You may need to adjust the worm drives a little to remove 'sticktion' (friction points) or excessive backlash that can't be recovered from.
From you very first image you're guiding didn't look too bad, but I would change from using "pixels" as units to arc-seconds - the pixels always look much better than the arc-seconds.
Cheers,
Mark