Is there an option for him to get a green card if he will marry a us citizen in a future after this devorce ?

Your post seems to imply your motivation is only a green card. Proceed at your own risk.

Should i choose withe based on N 400 instruction or leave it blank ?

Based on the instructions choosing white as his race would be correct.

How can A derived family member enter the u.s legally? under what conditions/ circumstances?

If as the principal applicant you are under 21, your sister could qualify for derivative U status if she is under age 18 and not married. If she...

Can I get a work permit if I'm illegal in the US?

It is possible you will qualify for DACA. Search on USCIS.gov or google it.

There are an abundance of records that would serve the need, including banks. Have you actually tried asking the banks and companies you posted...

Is it mandatory for a petitioner to file the form i 864. If I have someone who has enough income to sponsor

Yes as petitioner you must file an I864.

Do not send the information, but rather bring it with you to your interview along with her BC.

My question is Can I continue working after my EAD expires? (Since I have the EAC number from USCIS saying my case was received)

You are not permitted to work without an approved EAD.

G-1 is a nonimmigrant visa for diplomats. While you can go to school, you are not authorized to work unless there is a reciprocal agreement between...

What should she do in this case?

Forget this guy, and count your blessings.