Best to have an attorney review your case for an in depth analysis. Police records are not required but are good evidence to establish the abuse.

What remedy or status i can apply?

Very sorry to hear these circumstances. Visa overstay is forgiven through marriage to USC so this is the option available to you.

Yes you needed to or provide clear and convincing evidence of the first marriage. Best to work with an attorney for RFE response.

For interview preparation, best to discuss with an attorney in private given the nature of your aunt’s case. Good luck.

It can take many months depending on the track. Best to refer to the USCIS webpage for guidance.

Does i864A have to be submitted evidential document ?

Yes the cosignor and household member must provide the same supporting documentation at underlying sponsor.

My girlfriend from Thailand has a B1/B2 visa. How do I convert to a fiancé visa?

She should not enter on a tourist visa with the intention of marriage and adjusting status but that may be an option if your circumstances change...

Your opinion on filing VAWA cancellation of removal?

Yes you may do both but best to consult with your retained counsel as s/he knows your case best.

Does joint sponsor has to submit proof of domicile?

NVC may request proof of domicile if applying through consular processing but likely not through AOS.

No not if the reason is a divorce as you indicated.