Help with husbands narcissistic parents?

Hello! This is my first post, so please bare with me. I'm asking for help with a two-fold problem, so this may be long.

1. My husband and I have been together for 6 years, and the first maybe 3-4 were full of fights with and over his narcissistic parents. We've had NC with them for about a year and a half, but they keep texting/emailing him, and mailing him letters. They have always disliked me, since I haven't ever let them manipulate me and saw what they were doing from the get go, and have since made my husband see. One of their favorite hobbies seems to be to try and get the rest of his family to turn against him, and unfortunately have had relative success in that department. How can I help him get through this, and understand how wonderful he is?

2. my husband has behaviors that I had no idea were caused by his NP until recently. Like, when we argue he'll continue trying to explain his point long after I conceded. I guess I'm trying to find out how I can help someone who's been invalidated and manipulated his entire life understand he's in a safe place now, and that his thoughts and feelings are important to me? And what is the best way I can respond to not continue making him feel invalidated when he does stuff like that?