I would just go ahead with my own plans.....make the cake I had planned, birthday week is huge at our house, and the cake is a huge part, it's got to fit the theme! I wouldn't even bother saying anything about it, I'd feel bad as she's just excited I guess. I'd look at it as part of her gift.
As for the popping in, I've tried, DH has had a word many times nicely and not so nicely, it always goes back to the old unannounced- I was in the area visits, it annoys me, especially if we are on our way out, or we just got out of bed, but I no longer let it change my plans, so if I'm doing something or going out, I don't let it interfere, I keep doing what I'm doing. We also get the junk food, stuff we would never buy, dh tells them all the time, they still keep bringing junk, so I think get your DH to have a word, but in my case they slowly but surely always go back to their old ways!