Didn't think you'd be able to stay away!
It will be lovely to see you all again, and meet the new baby.
Sorry can't help much ref the powdered milk question, but I know that Valantis was always putting baby seats and booster seats in hire cars.....he's not got an office in San Stef now, but can be found online. But you also have the choice in the village of Dimitra and Manos, Tsaros, Olgas and Noula for cars, and I expect they provide the same service.
Also, don't know about baby seats in taxis....maybe Lesley from Corfu selections can help you on that one, as she know taxi drivers.