Use and Validity of JR Kanto Pass

We are foreigners and this is our first time to visit Japan with my family. From Osaka, we will arrive Tokyo on March 30, 2104 and will travel back to Manila on April 4. I am planning to get a JR Kanto Pass for us and use it on our trip to Gala Yuzawa Ski Resort and Mt. Fuji Trip and use it for the last 3 days of our stay in Tokyo - April 2, 3 & 4. Here are my questions:
1. When do we start counting the 3 days validity of the JR Kanto Pass. Is it from the time it was purchased or time it was first used? Assuming I purchase it on April 1, but started to use it on April 2, does this mean it is still valid until April 4?
2. We will be staying in Shibuya. Is the JR Kanto Pass valid at the JR Narita Express? Do I need to reserve seats?
3. Is the JR Kanto Pass valid at the circular line of Yamamote Line?
4. Going to Gala Yuzawa Ski Resort, which I understand is covered by JR Kanto Pass if I pay an additional JPY1,000, where do I take the train going to the resort from Shibuya Station, is it Tokyo Station?
5. Going to Mt. Fuji from Shibuya, where do I take the train and which line? is it also from Tokyo Station? What type of train is covered by JR Kanto Pas going to Mt. Fuji?
6. Can I buy the JR Kanto Pass in Osaka? Do they accept credit card?

Please bear with me as we are first time visistors of Japan and I am confused with a lot of train options available and we would like to spend wisely.

Mnay thaks for the help.