My daughter's niece can't say Rs. She calls my SIL Uncle Weed. Yes it's adorable, but he's not going to start calling himself Weed. Grown-ups continue to model the correct pronunciation and she eventually masters the R sound. She hears Reed, she just can't say it yet. Likewise, your grandbabies have been hearing Grandma, and it came out wrong. A cute mispronunciation. Soon mommaw will be gamma or maybe gwanma and finally Grandma. Might it be confusing to them to change your name at this point?
If you've been going by Grandma all along and even encouraging them to call you Grandma, I don't understand the importance of changing your name to Mommaw, or why the spelling matters, especially if it causes a rift between you and the person who will ultimately determine how much you will be allowed to see the kids. Would you really want your very name to irk your DIL?
You ask if we decided if Mammaw was probably ok...I think the point everyone here is making is that it doesn't sound like it's ok with your DIL, and she's the one who matters, unfortunately.
No matter what they call you, it's gonna sound sweet.