I don't feel the grandmothers behaviour based solely on the recount here is remotely twattish. I'm absolutely not trying to stick the boot into the OP there: I know it's possible for little things to be significant based on such minor things as a look, which is hard to explain via text, but if I am to take the posts at face value the MIL is getting a really undeserved kicking.
Again, said very gently, not only comes there a time when you have to move beyond the past, there also comes a time where it's vital for your own mental health but also that of your loved ones for you to prioritise the living over the dead.
I think when you've had a significant loss - and significant doesn't always mean in terms of blood or marital relations but it usually does - at a young age, it's easy to feel jealous, bitter, angry. I've felt all of these, especially the former, at various times. At university, I was SO jealous of my friends with lovely mums. I felt upset right up until about 2007/2008 and ten years passed and somehow it didn't bite quite as hard.
Losing my dad two years ago, when I was 32, was a wrench. I felt cheated - as if having had one parent prematurely die on me, I was "owed" the other one, somehow.
Yet as the months have gone by and I've experienced again losing a parent but this time as an adult, not a girl, I've come to understand so many things that I didn't understand fully back then. I've come to understand that the greatest gift I can give my children isn't photos or stories or anecdotes - though these are important - it's the very essence of my dad, and that lives in me. We looked alike - same wide smile and dark eyes, same short legs
and small mouth, but our characters really are identical. The easygoing and happy way of looking at the world, the clumsiness, the love of reading and literature, the pretence of being annoyed by dogs when really we both adored them. Realising that I am so very like my dad that he'll just never really go away has been the source of comfort. I don't have to go anywhere to find him, because he's right here.
Yet within that I'm my own person too, and my children deserve to be their own people. I hope when I have children, they will know they had a handsome, clever, thoughtful and kind man as their grandfather. But I also hope that while they will know this, their relationship with their hopefully living grandfather (and grandmother) will be loving and positive and will bring joy and happiness to their lives.
Being possessive about a child's love - which (sorry) is what this is - never ends well and is as pointless as it is harmful. We all have limitless ability to love and love others. Your little boy growing to have a loving relationship with his grandparents does not diminish your fathers role in his life, but your fathers role is in a sense done. His living grandparents have priority.
I wish you well.