A:AnswerThere are "cellular" models available too. If here doesn't have it, then you are guaranteed to find it over at Amazon and eBay. I've seen AT&T models over there :)
A:AnswerI believe that the taxes and/or shipping price will vary depending on where and how it getting to you. If there is a best buy location, you'll pay for the taxes, and if you meet the requirement from Best Buy, there may be no shipping charge.
A:AnswerIt is not included, but you can get one.
On the box itself (the back) it lists the contents. It fails to mention the spare SPen nibs, but doesn't say it has this adapter, and mine didn't have it.
A:AnswerYes, but assuming you mean the 3.5mm plug style you will need a USB C to headphone jack adapter (there are many variants including those that let you charge).
You could also use a bluetooth to headphone, or just bluetooth earbuds or a headset.
A:AnswerIt does not have phone hardware, but some Voice over IP services work (e.g. use CSIPSimple) which support voice, and some also support SMS, either through the app or the website. You will need wifi or another network connection.
There's also Skype or a similar service.