Awww, it sounds like you're working really hard for your Mum, well done.
No doubt the answer to the Wednesday problems lies with the grandmas again. Could you ask one of them invite her to pop in on them on her birthday? Then they could come back to the house together. Or what about a neighbour, could they help plot a scheme with you?
As for the present, it would be good to have a rough idea how old Mum is and how much you have to spend.