Answers to some of your questions:

I'm not familiar with the JR Pass. So maybe someone could help you with that.

- If we want to buy electronics, will the salespeople understand that it needs to be compatible with the voltage in New Zealand?.. or is there something on the product that we should look out for? Do many salespeople speak/understand English?

Yes. For all your electronic needs I would stick with going to Akihabara. There's typically English speaking agents at the stores around the area that are able to assist you. There's usually signs around the product you're looking for that states specific voltage.

- Is there any outlet shopping in Tokyo?

I don't know your interpretation of "outlet", but if it's anything like outlets here in the US...then NO! But you can generally do some bargain shopping at Ueno's Ameyoko street.

Can we take the baby pushchair on the rail network?..

Yes you can. But if you do, I would suggest to avoid rush hour as much as you can. which is 6-9 AM and between around 5-8 PM. All the hours outside of this time should be ok. If Tokyo is infamous for anything, it's the crowded trains. All

Most touristy places you can reach by taking the green Circle line (Yamanote Line). Goes around all of Tokyo. For the most part you won't need to deviate off this line. Maybe here and there..possibly.

- Since we are only in the city for 5 days, what would you suggest is an absolute must do?.. we are into art, culture, nature, fashion and electronics.

Nature- Ueno Zoo/Park

Fashion- Harajuku, Shibuya, Roppongi

Electronics- Akihabara