Buying ticket more than 6 months in advance?

Hello

I am travelling to Japan in 2024, arriving on the 22nd March in Tokyo Haneda airport.

Can I buy a JR Pass in advance now ? And if so, what date do I need to activate the pass by if I buy the ticket tomorrow before the price increase?

I will be in Japan for 2 months, and was only thinking of using a 14 day ticket/the bullet train in mid April, but I’m not sure if I can only purchase a ticket up to 6 months in advance.

Which means I will have to activate the ticket by the 28th of March at the latest if I purchase it tomorrow.

Also do I need the JR Pass to ride the train from the airport into Shimokitazawa? Or can I use the metro?

Finally what locations can you swap your voucher for a JR Pass ? Is it only at the airport?

Thank you !