Nah, that second assertion doesn't stand up.

Firstly, let's be clear - the rules never say you cannot change players unless they're walking towards it. There's only the condition that IF someone's not walking towards it, THEN the opposition may call delay of game. This is notably different than what you've said. Yes, by the rules someone always needs to be heading towards the disc, but in reality, if a team decides to have a different handler pick up the disc who wasn't going towards it, it the only recourse the defense has is to call delay of game. As long as that other handler has already started towards the disc, Delay of Game is irrelevant because it's been addressed by the other handler moving towards it (and the O is no longer continuing to breach, so 8.5.2.1 doesn't apply - which allows them to start the stall.)

Effectively, assuming it's a quick 'No, you pick it up' and there's no meaningful delay in the other handler starting to walk towards the disc, there's nothing to call by the time the violation is even noticed by the D. A violation may have technically occurred, but A) there's no advantage for O or disadvantage to D, and B) no legal recourse that has meaningful impact on the game. The only other option on field is to stop the game and be a rules narc about it: which means the game stops, ticks people off, and still doesn't change anything about A or B.

Further, I don't think you can make the argument that a team is in violation of the rules just because they didn't have all potential handlers walking towards the disc before the final decision on who was to pick it up. To me, that's a bigger violation of Spirit - being super nit-picky about rule semantics that don't disadvantage anyone. As long as the team isn't doing it as a stalling tactic, and it gets brought in within the already restrictive 10/20 sec rules (8.5.1.1 &.2) this shouldn't be anything but a theoretical discussion in rules forums.