Of course third party retail was a thing in BR days. Just delivered by high street travel agents AIUI
Absolutely, and the key word again is "Agent", nothing really changes.
Would GBR or TOCs go up against the Agents?
I suspect if it ended up in court the railway would be torn apart by lawyers with repeated examples of terrible failures by Government run organisations to provide accurate and resilient fare and routing data-tables and feeds.
Which does make me wonder.
Has anyone checked to see if there was a data-blip at that point? Did Trainline merely pass on dodgy data from National Rail?