I think your letter is lovely, personally i would not use the word "encumbered" as it does have some negative connotations. I think you should stress that you all love your granddaughter, and you only want what is best for her - and to see her sometimes!!!! Contacting the x has nothing ot do with the x, just a grandmorther wanting to see her grandchild for goodness sakes!
I am lucky that my partner is and always has been so understanding on what comes along with a little one - he knows that there is absolutely nothing between my x and i and we only contact each other when it comes to arranging visits, never for a chat! But, i am still in contact with his parents so that they do not mis out on seeing my little girl grow up. Your new DIL should understand that too!