Question on USAGE

Hi there,

I just received my JR pass last week. I am going to Japan from April 9-21. I don't plan on using the JR pass until April 15. Since I will be at Narita to purchase a one-way Suica and N'EX at the JR East Office, I can go ahead and tell them that I want to activate my JR Pass for April 15 or do I need to do this a day or two before I actually activate it at a local station in Tokyo?

Also, I was wondering if the pre-purchased SUICA/NEX package will work in Yokohama? Is the existing amount topped up in the Suica card sufficient to travel throughout Tokyo and to Yokohama?

Thanks