I was born in Calcutta in '74, moved to Australia and Portugal as a kid, and I'm no scholar, so I'm about to look like a very bad Indian! Plus I grew up in a Hindi & Muslim, Indian & Portuguese household and converted to Buddhism in my teens, so I wish I could be more helpful but I don't really know so much about Hinduism.
Here's a basic timeline that'll get you started in the right direction. http://www.bbc.co.uk/news/world-south-asia-12641776 Maybe you can find helpful info at the Indian Memory Project. http://indianmemoryproject.wordpress.com/
1. I don't remember so I don't know what different now. I know they take for ever, there are many rituals, and there's tons of prep. The mendhi (henna painting) is my favourite part.
2. Sorry. Don't know. There's no Indian American section in the citizenship classes/exam! But there are tons of us there now.
3. Hindus I know don't seem to have much of an opinion of Christmas one way or another. Nothing is open. They go to the movies or stay home. They/we love Christmas sales because it's a great time to get bargains :-/ and lights to decorate the house for Diwali :-).
4. It seems like there are more holidays than days of the year, but Diwali and Holi are the most important to my family.