To be frank, no, I don't think it would be fair to ask C to hold off on developing a new relationship. You and he have been together a year and things are still so tumultuous, so if he's supposed to wait for things to finally be stable, how long will that be? Another year? Longer? It sounds like he's been going above and beyond to provide you with emotional support and try to make this work even to the point of giving up your anniversary celebration to help D's insecurities. If, considering all he's given to this relationship, you insist that you get two primaries and he can only have one serious relationship, I can only see that leading to some sincere resentment over time.
What you CAN ask for is for him to keep you in the loop -- he should be the one to tell you if things are getting serious, not D.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.