Too many questions in one but let me answer them in order:
Yes you do need a visa since the US doesn’t have international airside transit zones see question: Do I need a US visa to transit (or layover) through an American airport?
You have been told correctly. C1 Visa is valid for the maximum period of 60 months, which means that if granted for that period you will be able to transit through the United States for 5 years.
This is a good question to ask a consulate. Normally a multiple entry C1 visa will cover you but it may happen that you will be required to apply for multiple C visas as I cannot find any definitive information regarding multiple entry travel through the United States.
Credit:stackoverflow.com‘
5 Mar, 2024
4 Mar, 2024