Out our club the view is usually taken that by shaking the opponent's hand, you are agreeing to concede the game. I don't necessarily agree with this but the view is taken that at that point the result is final and subsequent claims of illegal moves, loss on time and unseen defenses to mating threats are moot.
This can probably be interpreted as a local variation on 8.7
At the conclusion of the game both players shall sign both scoresheets, indicating the result of the game. Even if incorrect, this result shall stand, unless the arbiter decides otherwise.
Since we generally do not sign score sheets the handshake is taken as the crossing of the Rubicon, even at "normal" time controls. However, there are a number of problems.
Firstly, the handshake often takes place before the result has been written on the score sheet so the two players may genuinely disagree about the result of the game.
Secondly gamesmanship is given a greater opportunity since a so-minded player may make a checking move, claim mate and stick out his hand hoping to embarrass his opponent into a ill-considered "agreement."
Finally, there is no physical evidence that a handshake takes place. A signature on a score sheet is at least a permanent record that the player agreed to the game (although it's not perfect since tampering can obviously occur after signing).
Anyway, so getting back to the case from post #1 perhaps there local rule of a handshake somehow fixing the result in the same way as the signing of the score sheet from Law 8.7 is more widespread than just my local.
In rapidplay of course there are no score sheets involved so then perhaps the shaking of hands might taken as the equivalent role of fixing the result in concrete (the first two problems I mention earlier notwithstanding) but you still need some point of no return from which point the result is final. I can't see a justification from the rules that says shaking hands should be that point, it is may be an understanding which is out there.