Tak to an experienced lawyer.

How you filed I485? If so, you are safe. Otherwise, if your H4 extension is denied 6 months later, you will have been out of status for 6 months. In that case, you will be in a big trouble. Consult with a good lawyer about your situation and spend some money. Applying for an H4 visa in China might be safer.

10 days of grace period totally depend on immigration officers' discretion.