My adoption background in very similiar to yours so I won't bore with history
From personal experience (both my asister and I are in reunion with our respective bmom & family), the only way to bridge the gap is time.
You have done everything you can to reassure your aparents, and it is up to them to meet you halfway. It might not get much better.
My bmom and I only communicate via email and the occasional gift, and my amom is quite pleased about this. Amom never asks about my bmom. Amom and I have never been very close, we are so different from each other, and she is always so afraid of rejection she couldnt' be happier that bmom and I are taking our sweet time to get together.
My asister, on the other hand, is in full reunion with her lovely bmom and extended family. My amom has *tried* her best to be positive but she feels threatened, and like your aparents, adamantly refuses to admit it. Amom is ALWAYS asking me why my sister is doing this, has she seen them, did they talk recently, etc etc. Amom is always trying to interject drama where none exists. My sister only tells amom minimal info now, as it was getting ridiculous. She was wanting almost weekly reports! Amom and sister's bmom met early on in reunion and it went well, both moms wanted to meet the other.
I think what really annoys my Mom about our respective reunions is that asister and I are so much like our birth families in personality, looks, behaviours. I know my amom feels left out b/c of that - you can't override genetics no matter how much you try and it is *the* thorn in my mother's side.