My H1-b visa is approved and it begins on 1st october. I am currently on OPT-stem extn. Can I travel outside the US and return before october without h1b stamp?
Yes, provided that you have valid F-1 stamp. But I guess your COS will automatically become CP after 10/1?
That would depend on the 'status' with which your employer applied for you. If the status == 'AOS', don't travel. If status == 'Consular Processing', you can travel. Also, talk to your immigration lawyer.
AOS - adjustment of status, non immigrant status -> green card.
No. If you travel outside you have to stamp a visa in your passport. Your change of status was approved which is different from visa.