It took a little thinking to work through the algebra, but essentially the OP is proposing to travel A-C (with brief landing in B, but continuing on the same aircraft), then returning from C-A. This will earn the same as a direct return flight A-C.
If however the traveller waited in B and caught a later flight (a problem if the service is daily or less frequent!), or caught the next day's flight from B-C (the same flight number but different date) the points would be earned A-B, B-C, C-A.
So there is no advantage in taking the ABC flight over an AC direct flight if all on the same day (unless it gets you a full sized aircraft with reclining seats!)