Hi Neil
I don't see the example you provided as being 'unusual' because what you have done is pulled forward into a column with one of the elements on the flank of the formation. I understand you left element A behind but given he is not going into the column (and cannot*) you haven't actually provided a situation where you form a column behind an element other than one of the flank elements.
* Element A cannot join the tail of the column (per your scenario) if you read through the group movement rules (nearest elements fall in behind), so is not a part of the column.
Another (absurd?) scenario is 2 elements of Cav side by side, and you wish to pull into a column at say a 20 degree angle to your front left. If you lead with the left element then the group move has gone short (both outsides corners move less than 240 paces), but if you lead with the right element then it hasn't.
Andrew