I come from a different place, so I might be projecting. (I did not talk to my dad for years. Our relationship now is much more different.)
I do not understand why your mom went to the funeral. There is hard feelings between your dad and her. It was HIS mother. She should have been more respectful and visted when he wasn't there. If she has been part of that family I am sure she could have manage a visit to the funeral home when he was gone. Or said her good byes at another time. No matter how much part of the family she has been, she should have put her needs to the side for him and HIS family.
Remember your dad is greiving. Grief can bring up a lot of raw emotions from other situations.
My mom did interfer with my dad's parenting. She was minipulative, emotionally, and verbally abusive. It took me many years to see how my mom created a lot of the situations that she blamed my dad on.
1. My dad wanted to co-sleep. She wouldn't allow it, it would spoil us
2. My dad hated CIO.
3. MY mom would tell my dad he was a failure because we would cry to long.
4. Many little ideas that he had were wrong. I could go on on the ways she did interfer with the relationship. Her way was the only way (this does not absolve my dad's bad behavior).
5. Your mom might have interfered more than you realize. Little ways of her disrespecting, belittling, or countering his parental decissions.
I have a feeling your dad has felt for a long time disrespected by your mom. Then your mom showed up to his mom's funural. He felt even more disrespected. She did not take a momment to think about how that would make him feel.
You gave exsuces why you didn't make time (some what understandable) to focus on him when his mom died. I can see how he feels a kick in the face. It is not what you intended but I can see how he would feel second when he needed to be put at a higher priority.
You tell him in his grief of loosing his mother (your grandmother) you can't give him the time. Personnally "I can give it attention right now" would have been a kick in the face! How would you feel if your mom died and your child/husband said "I can give it attention right now." I would be hurt!
You could talk about how your first Christmas in your new home is more important that talking to him. You want him to be excited for you but you can be sorry for/with him-- that is how he read it. I honestly would feel majority crappy if anyone one would tell me that while I was grieving.
You mention you manage an out of state trip but to his home, you could only think of her often --- I can see how he feels that these other people were more important than him, his mom, and their grandmother. His mom is only an often thought.
I do not know all your entire story but that letter that you wrong, putting me off until February, after my mom died would have hurt. For me, it is saying you could not stop for a few minutes to grieve with me about your grand mother.
I am wonderding if he feels that you treat him with the same disregard as your mother did. This might no be right but that is how he feels.