2) Shopping for kids is great, with many American and European brands although beware: Japan is very expensive.
3) For short distances, I would take a taxi. There are no laws regarding car seats and it's much more convenient than taking the train, especially at rush hour.
4)Narita airport is a good two hours from the city. You can take the train if your destination is on or close to a train line, or you can take a bus, which often takes longer due to traffic. Plan accordingly, especially on your return. And, as always, pack many snacks.
5) The Japanese are insanely polite and very fond of children. If you are stuck, gesture madly and ask for help - someone is sure to give you a hand!
Sayonara!
Other posts in the Tokyo series:
Tokyo, Japan - Karaoke is baby-friendly!
Tokyo, Japan - Tokyo's Best Shopping for and with Baby
Tokyo, Japan - New York Grill: All High, No Chair
Tokyo, Japan - Cerulean Hotel: Pretty Good!