I switched from puppy to adult food at about 5 months of age for both my pups. Their breeders said 4 months was an OK time to switch. If you got her from a breeder, you could ask the breeder. My vet also said at 4 months a straight up adult food was fine. Many foods are All Life Stages foods, so if you switch to one that's an ALS food, then both puppies and adults can eat it anyway.
I don't have suggestions for Gigi and the 9 year old, just that I remember those days well. In her eyes, we just got dumber and dumber the older the kid got until about college age, probably after college. All of a sudden they decided we weren't as dumb as they thought.
My main thought would be, in the house anyway, to have a spot that is her "place", a bed or rug or towel, and when she starts "intervening" in your actions, stop, have her go to her place and stay while you continue whatever you were doing. Remove her from the immediate action. Gigi, this does not concern you, go lie down.
On walks I've tried taking along yummy treats and if the dog starts getting all riled up, I tell them to "watch me" and give them a treat when they turn to me. If I have treats with me, I can get them to ignore the other person until they pass. It takes a lot of practice and I have not been consistent enough with my training for it to always work. (True Confessions) If I don't have any sort of treat, they don't watch me for long!