Wow, vincevoltaire, sorry for you for your troubles. And sorry for some of the responses. All those shoulda coulda woulda suggestions are because air travel has become ultra-complicated... so for a traveller who doesn't live on FT... how could you know all this? They / we know to link tickets, to check-in at 24 hrs, get seat selections, to ask the FA for seat possibilities, be proactive, offer alternatives to them, check what they do etc. etc, ad nauseum.
They / we know because we were exactly in your shoes once upon a time and, because experience is a great teacher, we learned by doing some of the exact same “errors” you made. In fact, we still do when we get lazy or busy... because air travel is excessively complicated now. That is why we seek help from AC agents, to help us... but as your story goes, sometimes not. You have valid reasons for thinking AC let you down.
I bet you link your tickets in the future