I-131 AP RFE - travelled outside of the country during AP processing

That RFE needs to be responded to - no matter what.

Do i need to send a fresh I-765 form along with the supporting documents as well as the cashier's check?

All such rejections come back accompanied by a Rejection Notice - with instructions on what to do next. Follow them.

How much longer until my husband gets his interview notice for his Green Card ?

For Dallas, you could easily wait another 12-15 months from now.

Is it possible to upgrade the filed petitions H4 COS & H4 EAD to premium process while applying H1-B Extension?

Go ahead and "tag" - remains to be seen if the PPS Unit will move on it or not.

I-485 interview scheduled before Final Action Date?

I agree. Wife should attend the interview. She will most likely be granted this month.

Will I type N/A in the address section on Part 8,N-400 form, If I am unemployed?

"None" for the employer and "N/A" for the address are the way to go here.

Travel on Approved I797 when company is merged

If the employer representing the employer is no more, then you should immediately schedule a phone or Skype consultation with a neutral attorney.

Going Back to Previous Employer for Green Card

Hopefully you do understand that once the I-485 is approved you will need to start working for that employer - full time.

I am an Austrian citizen ..how long would it take for F4 visa?

Your brother's petition on your behalf will likely take up to 16 years to mature and have an immigrant visa number available.

This question is a matter of state law. Research the law in your state about this.