No, there never has been anything about writing off Harry if William and Catherine are targeted. William certainly isn't going to appease the Sussexes with Catherine being targeted and I can see him telling his father to go jump if Charles wants to William to bend to Harry.
I agree with you that the only family members interviewed for the film that I can think of are Jack and Eugenie, she not a family per say, but Fergie, maybe? Well, an ex-family member still with her ex-husband, I guess.
Yep, if it is Jack and Eugenie, the door to Sandringham will be closed for Christmas holidays.