Answers above are not accurate, coming from experience:
...both as an international (US) and national (Canada) buyer.
1) Depends on the shipping you choose
- When buy, it asks you if you want to wait until everything is ready to ship in one box or to ship separately.
- If you pick cheap shipping, sometimes you don't have the choice and they will send you whatever is cheaper for them.
2) On every item's page, it will show the seller. If you buy "from amazon" it will be a better experience, overall. It's usually quicker and tracking always works.
example of something I bought:
You will see it shipped from Amazon. Fun fact: I bought this from the US site because they didn't have any on the Canadian site. They charged me duty but after I received the item, they refunded the duty back onto my credit card.
example of something I bought:
This watch was bought from MrWatch. It took longer, there was no tracking, and if you have to do any refunds/complaints you need to deal directly with the seller.

Most items are from amazon. You usually only buy from other sellers when you buy used or amazon is sold out.
3) As far as I know, no. They might have a coupon/code system, not sure.