Is documentation of parental relationship required on N-400 to prove relationship to step-children? If so, what is accepted?

There is no reason to prove that relationship in N-400 proceedings.

You will need to consult with a Board Certified immigration attorney for answers to these questions, especially because we do not have enough facts...

Laid off on h1b due to covid-19, change to b1/b2 visa?

It is highly unlikely CIS will approve that change of status since you must prove why you need a visit even though you have already been in the US...

Got H1B extension RFE after I got marriage based GC, questions on that situation

The next step is to speak with the attorney who is being paid to represent you.

If you were older than 26 when you received your green card, you were not required to register for Selective Service.

Under the current circumstances, can an F2 status holder receive work authorization due to hardship? Spouse of the F2 is on opt.

There would have to be a change in circumstances with your financial situation for your F-1 support. The pandemic by itself does not qualify.

Can I file for Removal of conditional status on family based greencard with an expired passport?

Yes, you can file for removal of conditions with an expired passport. You should also submit documentation of your pending extension.

Documents to send with I-751?

There is not nearly enough information about you and your spouse, let alone your daughter. And turning 18 has no impact on her case. You need to...

Process for EB1-C to Greencard?

You need to consult with a Board Certified Immigration attorney. No one has enough information to advise you, nor could they in this format.

Is it legal for a local city police officer to come to your house and ask for your immigration paperwork to prove you're legal?

It is not legal for anyone to do that unless the owner or renter of the home gives permission or the officer has a warrant.