1. You can stay in the United States. You just cannot work. So while your timely H-1B extension is pending you are not illegal, even after 240 days. The only thing is you will not have work authorization. You can also ask for an expedite at that point of time. It is worth trying. Even if the premium processing is not available expedited processing is still available.
2. You don't have to re apply for the extension. The pending case would work. The only thing is since you left while the case was pending you would require to get a visa stamping and come back again. More...
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