I find it very hard OotW, to formulate a helpful reply.
When your post first appeared I had to look up NPD and now I've had to look up being an Empath since I only had the vaguest idea of what that meant, and only then because of the stem of the word,.
There are obviously family issues here that are unique to yourselves and I for one wouldn't want to comment on the background to it.
The only thing I think I can helpfully say is that your confusion, though understandable, is perhaps coloured by your own personality.
Sometimes when somebody is diagnosed with cancer, they sometimes feel that the only way they can cope, is to "shut out" those nearest to them because they cannot take on the hurt and pain their own condition is now causing others. It's enough to cope with one's own fears without taking on board everyone elses.
In the world of prostate cancer, there will be many who need the love and support from family and friends - there will also be those who cannot bear the thought of people feeling sorry for them. We are all different.
I'm sure that even those who would rather carry their Cancer burden by themselves do not see it in any way as controlling others or being selfish.
We have so many on here who struggle just to get through a working day due to fatigue or depression etc caused by their brush with this horrible disease. I am pretty certain that not one of them could see themselves, or those of us who offer support to them, as "tortured internally to have to torture others with their behaviour"
We have the occasional "spat" on here where comments get made or taken in the wrong way. We don't analyse the whys and wherefores. We give and take because we all know that what each one goes through on their journey can lead to distress and short temper occasionally.
I'm not really sure what it is you are asking of us. Is it your FIL who has the "Narcissistic personality disorder"?
Are you saying that because he had NPD he is incapable of sharing his cancer information with his son?
If he won't relate to his son (whatever the history of that is) then perhaps your husband could make the approach on the basis that PC is often familial and your husband would benefit from being checked by his family doctor for PC so his father's details would be beneficial.
I hope somebody else will come along and perhaps give more clarity to your post. I'm sorry that I can't be more helpful but quite honestly I'm a bit lost