yes, you will get new I-94 with your approval.

salary is based on your job type, locations and so on...

well, if you want, you can leave usa and come back to join your company starting from october 1st, this way, you dont need to worry about new I 20.  trust me, getting H1b from nepal is very easy with your H1b approval notive.

i am not sure about your 4th question's answer.  my understanding is, if you get new I-20, your opt will automatically expires.  but not sure from what date.  i am guessing it will be effective from starting date of your I 20.  HTH.