I am in the exact (ditto) situation as yours. However I did manage to find out the below.
1. Talk to your employer and attorney before you final your trip to India.
2. Because you are already in US; your H1b petition would have been filed with COS. Now if you travel to India before or after your approval, Your COS would become invalid. And after your return back to US on Your H4 before october, your employer has to file for your COS with your new I-94 which will cost some $. COS approval time again depends on USCIS, like 2 to 4 months may be. Check with attorney if you can file COS with premium processing so that the joining date doesn't get affected.
3. Else you can do consulate stamping of H1b at India and travel back to US in H1b on Sept 30th.
Hope this helps.
P.S: Because of the above issues, I have still not yet finaled the trip :(