A:AnswerIt does include the Samsung-brand stylus, amazingly.
Connect to your Android device, and it has whatever service level your cell carrier is providing in the United States. For US models, it works the same way when traveling abroad. Of course, if you have a US cell carrier and a US Samsung S8 Ultra tablet, you will not automatically get the performance of the EU and AU versions when using it in those regions, but the EU and AU versions perform the same with EU and AU cell carriers when in the US, as is pretty typical.
A:Answerno charging brick is included. you get a USB C cable.
Yes you can use your old USB C charger.
charging time completion will differ from you charger wattage.
not sure about the warranty question. you might want to check with the Best Buy team.