H1 B Transfer and Extention without travelling to USA,Do i need to go for stamping after H1 Transfer?

Hello All, I have got H1B stamped from company A in 2015 (valid till Aug 2018) but did not get the opportunity to travel so far. I am still in India working for MNC B. Recently i got an offer from one US based company Y and my transfer got approved for one Year (even though they applied for 3 years only one year approved).

My questions are below. Thanks in advance for your time.Highly appreciate all your answers.

1)if i want to travel to USA do i need to go for stamping again since there is a employer name change and Date extention?

2)What are all the documents i need to travel USA?

3)How long i can stay in USA after my H1 Transfer/ extention period do i need to comeback to india?