I would say that her statement speaks more about her than anything else. You have written in the past about her attitude and behaviour to you (in particular), so I wonder even if you had a child naturally would she accept it. I would definitely go down the adoption route, how could you deny a child the possibility of a loving home because of one bitter woman? To be honest, given the way she treats and speaks to you I am surprised that you even consider her opinion at all.
If she want's nothing to do with a prospective grandchild then fine, I see it as her loss. I certainly wouldn't wait for her to accept the situation, she seems to almost enjoy the fact that she can inflict so much "pain" (not the right word, but you know what I mean) into your lives, and that you actually humour her. I hope it doesn't affect your chances of adoption should you chose to do so. I would say love is more about quality than quantity and, by the sounds of it any child you might hopefully adopt will certainly have loving uncles, aunties and one grandparent, that is sometimes more than natural children have.
I would concentrate on what is right for you and your husband, and brush off her remarks as those of a bitter and jealous woman.
I wish you every success if you do decide to adopt, it is not always an easy decision to make.