I am on h4 visa but my visa got denied and mtr as well due to non attending biometrics.

No other alternative left but consular visa application abroad. Make sure to timely depart.

I am filing I 751 and it asks for certified court document.

A legible copy of the original will do in this instance.

Need to have a valid passport to cover the period sought. No exceptions.

Can you apply for extension of stay(i-539) when one application is already pending?

If the limit is about to be reached and wish to remain beyond, then need to file a second I-539 - timely.

Depending on more facts not available here, you could qualify for either E-2 or E-1. Analyze with a lawyer in private & come out with a successful...

Good idea to bring such documents with to the interview but not volunteer to produce them until and unless asked.

If I am going to visit a friend in USA who is gonna take care of my expenses can I use his or her bank statement ?

You could use your visa sponsor's bank account all right, however, there is much more to you being eligible to be issued a visa than just that.

Dubious at best, with such a meager company presence in the USA.

My F1 OPT expires on Feb, 2021. And I'd like to start my PhD from Fall 2021 (starting from September 2021). How can I stay ?

You will need to apply for a change of status to visitor to cover the period in between.

You need to list your full previous employment history.