Yes, that's correct. It's the train company that first causes your journey to become materially delayed that is liable. We've heard of previous cases where claims have been batted from pillar to post but ultimately you are entitled to bring a claim with either company and if this is the "wrong" company, they will forward it to the "right" one.
No; you claim on the basis of the delay at your destination. If operator A's train was late but they did not cause you to miss your connection, you would claim from operator B - but as above, if you did claim from the "wrong" company, they will forward the claim to the "right" one.
You can; this is also why the claim forms usually ask you both the details of your journey and of your ticket - as the two may be different. I've had no difficulties making claims where I've broken my journey and indeed it's somewhat rare that I am actually travelling from A to B on a ticket from A to B!
