Thank you KML, DnJ, Scout and PLC... it is really supportive to process this with you all.
H hasn't brought up the issue again, despite the fact that we interacting a few more times yesterday. He was distant and tried hard to 'ignore' me. I won't say anything unless he brings it up again.
Where I need continued help/advice/support is how challenging it is for me to not 'fix' this whole situation. Not so much in the specific instance of the birthday party; but in the broader sense of trying to build a roadmap towards friendship and mutual respect. I can see that H is not going to instigate this, nor offer an olive branch. But I know that he would likely respond positively if I were to do so.
But that feels like me reverting back to old patterns in our R and I want to do things differently now. I can feel the seeds of change in myself and my way of relating to others. And I am trying to convince myself that if me doing things differently leads to the final demise of our R, then it wasn't a R worth saving anyway.
How do I navigate this in terms of standing for and trying to 'save' my M and/or build a post-M friendship with H?