Peter,
IMHO you need to analyse the images - check and confirm the direction (relative to the scopes and the SBS) and magnitude of the drift, before assuming a worse case scenario.
In a SBS the "cone" error is just an up and down adjustment of either the tube rings or the saddle(s) - yes this can cause field rotation but no worse than you'd probably get using a piggyback system - what's the difference??
Do you have any prior images with the Vixen that were guided with a piggyback system? (ie not OAG etc) Do they show any relative movement?
Once you know how much movement is involved and the direction, you can home in a bit easier on possible causes.......
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