In the late 90's / early 00's, following the Asian financial crisis, it was a lot cheaper to buy fares SE Asia-LON-SE Asia than LON-SE Asia-LON, so I once bought a single to Bangkok, and for the next 7 or 8 years bought all my return tickets ex BKK/SIN/HKG/DEL. I was saving a lot of money, but had to be pretty confident of my dates up to 6 months ahead. When I discovered that under the old (now completely abandoned?) arcane fare rules, and the concept of Maximum Permitted Mileage I could tack on a side-trip to somewhere warm and pleasant in Europe at no extra cost, that was the icing on the cake, giving me two 'free' trips to Europe every year on BA.

One year I simply couldn't make the time to fly my side trip to Malaga, but it seemed clear from the way my (paper) ticket had been issued, that this was a side-trip on a separate booklet, so I didn't worry about any consequences.

A few days before starting the return leg back to Bangkok, when I went online to check if I had a seat allocated on the first sector from London to Dubai, I came across a big red cross against that flight on my itinerary. I 'phoned BA and they explained that while I had regarded my trip to Malaga as a side trip, the way the fare was constructed was as as a flight from Bangkok to Malaga via London, and as I had no-shown for the LON-AGP sector, they had automatically cancelled that flight and all further flights on the itinerary.

I spent some time on the 'phone seeing if there was anything that could be done. The person I spoke to was very understanding, but said it was simply not possible to reinstate the cancelled flights, and I would have no choice but to pay for them again. He did say however that although an automatic message had been sent to Cathay, advising them that they should cancel the final sector: DXB-BKK, it appeared that Cathay hadn't (yet) acted on that, and he said he would send a message to Cathay asking them to protect the onward flight from Dubai, although he couldn't be sure of the result.

As it happened, the flight from London to Dubai was very full, no seats at all in First or Business, and I ended up paying an extortionate amount of money to fly in PE, all the time wondering whether I would find, when I got to Dubai, that I didn't actually have an onward flight to Bangkok. Luckily Cathay hadn't cancelled the flight to BKK, and I breathed a sigh of relief that I didn't have to pay a further extortionate amount to purchase a last minute single fare from Dubai to Bangkok.

I had clearly screwed up; I accepted that BA were perfectly entitled to cancel all remaining sectors when I no-showed; and I felt very grateful that someone at BA intervened to ask Cathay to protect my final flight on the itinerary. I was left feeling pleased that I had got away with only a partial debacle, whereas the OP has clearly suffered a complete debacle, but I accepted the general principle, that I had stuffed up, and the rules are the rules.

While it might be interesting to conjecture how the courts may regard the potential unfairness of some of the CoC, I really don't think the possible, but unlikely, upside of taking court action is worth the hassle and the stress, not to mention the continued inability to get on with one's life, that would be involved.