Do you think that he will be more likely to die or fall ill if you divorce him, or less likely?

In general, I think that it seems that you, for your own sake, should not file for a divorce, because objectively, it sounds like you really do need the current arrangement and the security that comes with it. Also, since you write so clearly and to the point, it appears that you are mentally "with it" and OK - you do not sound like a person whose self-esteem has been eroded. Even though you wrote that "As a result of his narcissistic personality disorder he has completely eroded my self confidence/esteem and filled me with fear, confusion, shame, etc.", it does not sound that it is the case. I will not get into detail, but just comment on your mention of "confusion" (in bold) - you do not sound confused. Quite on the contrary, you sound totally OK. Maybe your realization of the effect the narcissism has had on you has already allowed you to negate that effect. Possible?

You do write about the negative effect of narcissism on one of the children who now needs therapy for it. But, even in the event of a divorce, the child will continue to interact with the narcissist father and be damaged by such interactions and need therapy to counteract the damage. So that does not seem to be a factor in your decision-making, unless it can be proven that less contact with the narcissistic father will dramatically improve the chances for your child - dramatically enough to outweigh the damage that being uprooted from current private schooling can cause.