It could very well be a combination of things that culminated to where you are now. Your concerns are completely understandable.

Narcissists aren't known for aging gracefully. Combine a predisposition toward those behaviors with life stress, age related concerns, health problems, and you'll potentially end up with a powder keg. It probably did get worse over time- approaching the "collapse."

My DH definitely had some unhealthy aspects to his personality from day one, but they were hardly deal breakers. He worsened over time, then developed some significant neurological problems - which the doctors say aren't a problem, because he's too young to have neurological problems. So who knows where the personality issues end and where the neurological problems begin. I completely understand your question about covert narcissist or damaged individual who needs help.

The only thing that can be done is to live a good, healthy life of your own, have decent boundaries, and decide how much you can and should give in the event that you choose to help them.