?when should I inform about my divorce to uscis Already filed jointly i751

Yes if you are intending to transfer your case to a waiver based filing due to divorce.

I will submit evidences for my request to extend. Am I staying legally if Im still in the US after the deadline of submission?

Yes you are legally in US until decision is made on your case.

That is your personal decision but if you decide to, proceeding with this during the naturalization process is best.

No you must be a USC to petition for your sibling.

If you file based on your fathers petition, you must maintain status in order to adjust status. Best to consult with an attorney for guidance.

Will unemployment benefits harm my immigration process?

No receiving unemployment benefits should not impact subsequent filings or eligibility to petition for someone. You may need a cosignor if you do...

After change of status rejected because of fee mistake , can I file another case?

Yes you may but you must be in status to do so.

My green card is up for renewal in Jan 2021. Should I apply for renewal or apply for citizenship given the Covid situation?

If you are eligible and have the desire, good to apply for citizenship.

Tracking number denial notice

No generally USCIS does not provide tracking info for denial notices.

No the I485 must be approved to file the I751.