Another issue is that B doesn't understand that if he kills himself, he kills all of them. Whilst I understand it is his body when he is in it, he doesn't grasp the concept that it is 'shared', I guess. Do I tell him? And, if so, how?
Yes, tell him, unless you're pretty sure it will trigger him to self-harm. If he's old enough to talk, he's old enough to gain a sense of being in the same body with others. If you can talk to him, I would say that the body he harms is also his own. I don't think that's the most effective argument because it's obvious yet often ignored in the logic of the self-harmer. State that if he harms the body, he will be harming innocents, littles.
From reading, it seems to me the most common problem is with the host or one or more adults (or just oldest ones) in the system, not with the littles and certainly not all of them. I've often wanted to end the life, as have others, but when faced with the inescapable fact that it would also affect the most innocent ones here, it gives us tremendous pause, even when we know they are us.
It might be important to get B to talk about why he wants to kill the body. Is he killing himself, someone else, all of them? Why not hold out for a period of time, what's the pressure for now or soon?