Having a pending case with USCIS shall I apply for a change of status from B2 to F1 or leave and get a visa appointment outside?

Your optimal situation would be to secure the F1 stamping outside the country.

Would this be possible?

Yes it may be possible. Talk to a qualified immigration attorney in your area please.

You can only work with a valid EAD and valid H4 status.

After admission of the I-824.

The process can take 5-6 months plus. NVc will first process and then the consulate will set the interview.

I129 approved however I539 got RFE, please help

Talk to a qualified immigration attorney to respond to the RFE. This cannot be done online. Good luck

How long will it take my resident mother to bring my unmarried sister over 21 to live in the USA as a permanent resident?

We do not have all the facts here. I suggest sitting down with an immigration attorney to find out her options.

Can I gain citizenship if I marry an American citizen with a criminal record?

If the marriage is real then you may be eligible to secure LPR status and eventually citizenship. Please talk to a qualified immigration attorney.

Why does USCIS say I do not meet the 125% income requirements?

You really need to have a qualified immigration attorney review the documents, I864 and help you with the RFE response. This cannot be done online...

How to address a mistake on my N-400 during the interview next week?

Just amend the N400 and inform the officer at the interview. You should be fine. Good luck.

Employment change after taking a H1B visa appointment

Please talk to the immigration counsel for the company. They are in the best position to assist you. The online forum in this case is not.