Can the covid-19 childcare grant provided by state revenue departments be considered a public charge by DHS?

No this would not amount to a public benefit.

Submit by paper and indicate N/A where appropriate.

Your case may be eligible for humanitarian reinstatement. Best to have an attorney with you to make such determination.

What is the first step to marry an american citizen

Are you currently in a good faith relationship with USC? First step is the marriage and filing the appropriate paperwork.

The previous marital history will definitely impact the subsequent petition. Best to have an attorney consultation for guidance.

New marriage to a US person and advice needed

Yes the previous marital history will likely impact the subsequent case. Best to have a consultation with an attorney for guidance.

How can I determined what Fraud and Misrepresentation was committed after I received a letter from USCIS accusing this?

For a complex case such as this, it is best to have an attorney review your case.

When to submit I-693?

No you bring the medical with you at time of interview not biometrics.

Do I need to submit divorce papers to file I130 for my stepchild?

No if not submitted at time of filing, USCIS will request it through RFE.

When should I apply for citizenship?

It appears you may be eligible to file in 01/2021 through marriage to USC.