Travel to India while on a valid H1-B and a transfer to other company approved

I have job from my company and have another offer from company B, B will apply for my transfer in premium. I have to travel to India for 15 days for my sisters wedding and the joining date for the next company is 10 days after that. Company B is unwilling to move the joining date by much. But I have around a month before my travel.

My question is:

  1. Can the transfer of an H1-B visa be completed more than a month before joining if done in PP ( assuming no rfe as it’s the same position and both companies have very very mature process

  2. If the transfer is approved and if I have not resigned from my existing company do I need to get the I 797 from new company. I will not be on the New companies payroll till 10 days after I come back

  3. My visa is valid right now till Dec 2018