Honestly I was fine with kaidus maincalling as coach because of how it came to be.
What you should compare isn't Crowns with kaidus maincalling on coach vs Crowns without kaidus but rather Crowns with kaidus maincalling on demo vs Crowns with kaidus maincalling on coach.
I'm pretty sure kaidus would've played if he could have done so. How much of an advantage was not having to play while maincalling? I don't know. Would Full Tilt have won if kaidus had played? I don't know. Would froyo have won vs Crowns if kaidus had played? I don't know.
What we do know is that without kaidus maincalling Crowns simply doesn't work. It was built around this. Without kaidus maincalling they had to switch Stark and kaptain so Stark could maincall and I think we can all agree that didn't work nearly as well.
So for the sake of getting to watch great TF2 I'm fine with it just this once.
However it should be banned as soon as possible. For Crowns it was the only way to keep up their level of play, maybe it was a slight upgrade. A new team that isn't depended on an injured maincaller could just get a straight DM upgrade on one class while improving their maincalling a bit at the same time.
Coaching between maps isn't preventable. Coaching between rounds wouldn't be possible outside of LAN without ~2 minute tactical pauses between each round or plugins. Allow it or ban it but be clear about it.