Trying to change my ex CAI DONE5. The airline that issued the ticket and has made previous changes now tells me the below is not a valid routing because I am "backtracking" by leaving Europe, entering Africa and reentering Europe (CAI-xLHR-CPT-xLHR-BKK, etc. then returning to Europe). Have not flown 1st segment yet but the airline doesn't seem to care.

CAI-xLHR-CPT-xLHR-BKK-KIX-HKG-AKL-SCL-MIA-LAX-xMAD-KEF-xTXL-DME-xDOH-CAI

The airline doesn't seem them to be concerned with any other part of the itinerary so I'm not bringing it up. Their only issue is the "backtracking" from LHR-CPT-LHR. I told them I can reenter Europe solely to transit but they say no. Who is right? What rule specifies I can reenter Europe for transit? Previously the same itinerary went CAI-xLHR-JNB-xDOH-AKL, etc. returning to Europe, but I had to get rid of Africa for personal reasons and now trying to add it back in is tough. They tell me that I cannot go LHR-CPT-DOH-BKK (going through DOH instead of LHR) because that is backtracking also and I won't be able to return to DOH at the end of the trip! I'm trying not to complain too loudly because the airline is willing to make changes without asking too many questions.