Filing Second B2 Extension if USCIS response awaits for first B2 extension and extension date also passed.

Yes the stay will be legal while they await the case processing.

How do I apply for immigration to USA for my family?

You may file for your family in different visa categories based on your US citizenship. Best to consult with an attorney for guidance.

Can I add my stepson to my husband's petition?

You must file a separate petition for your stepchild.

For form specific inquiries best to use the form instructions or consult with an attorney for guidance.

The US recognizes dual citizenship so likely not.

3 years from the date your spouse become a USC.

Can one submit to request that a k1 visa case be expedited when it is still at the level of the National Visa Center (NVC)?

Yes you may request expedite processing with NVC.

There is no obligation difference, only that you are titled differently.

You may indicate EWI based on information provided.

Yes but you will need to file Form I824 to have the case transferred to NVC which takes some time.