If it were an unauthorised connection, eg. crew/dispatcher waiting for passengers changing platform without prior agreement or permission, it goes to the relevant crew/station team or, in some cases, the operator in general, regardless of the cause of the platform change. It is a new primary cause in itself.
I broadly agree. I think it is easy to look at the issue in isolation while losing sight of the bigger picture. Each case is different, and should be considered on its own merit. Even if a service is only every 30/60 minutes, if, say there are long single-track sections ahead with intensive services over them, the delays will almost certainly snowball, and very quickly too. Good customer service is not just about those people wanting that service, but also people already onboard and others around the network too. Holding the train so that all customers can make their way onto the train isn't always the answer. Saying that ime provided that the crew are aware of the change, holding a train for an extra 10/20 seconds when people can be seen or heard running down the stairs is mostly unlikely to be an issue except at the height of peak periods in very selected parts of the network.
Prevention is far more effective where possible. Granted at certain locations it can be a tad tricky, such as Barnes as mentioned upthread due to the proximity of the crossover, but regular offending stations should be fairly well-known. In my opinion at known problem spots, staff should be readily on hand to provide assistance, and be empowered to authorise alternative transport should the result delay be an extended one.