No question the game clock starts on the 'ready.'
I also thought the play clock should be a 40, started when the ball becomes dead. But, what purpose does 3-2-4-c-6 serve, if it doesn't apply here?
Perhaps I am giving too much credit to those that write/edit the rules, but, historically, the editors, in particular, have been very careful with rule language. If 3-2-4-c-6 was to only apply to an award of a first down to B, then I think they would have said, "Team B first down." But, then, I also would have thought they'd have said, "Change of team possession" (not just change of possession - there is a difference). Strictly as written, we'd go with a 25 every time there was a simple change of player possession during the down! Obviously, that is not correct.
So, at this point, I'm not sure if an 'A-B-A' is a 25 or a 40 second play clock. From experience, I would like to see it be 25. We are going to stop the game clock to award A a first down. While that is no different than if they simply advanced for a first down, the difference in an A-B-A situation is the fact that such plays
1) tend to create a much greater intermingling of opponents when the play is over (requiring time by officials to ensure 'separation');
2) players tend to be scattered over a greater area of the field (also requiring time by officials to ensure observe and ensure there are no improprieties until they get on their respective sides of the NZ);
3) game officials have to move greater distances to handle and spot the ball;
All of these 'tendencies' almost always result in an increased amount of time needed to get the ball spotted, often pushing 20 on the play clock. Teams tend to be slower in makiing personnel changes, and getting plays called, which results in a much greater risk of a delay foul.
Such downs are not much different than a scrimmage kick down (which, by rule, is followed with a 25-second play clock, regardless of which team will next put the ball in play). Since Team A can't snap the ball until the R signals - after the ball is spotted - it would seem that it is everyone's best interest to go with a 25 every time there is a change of team possession during a down.