B1 visa temporary extension

Yes the I539 is the correct form for filing an extension.

What's the average cost to apply marriage based green card?

Best to directly contact attorneys for their fees as fees vary and we are not authorized to post such on Avvo.

You may indicate this status or file by paper if option not available.

B2 visa extension RFE on a list of activities

See previous answer to previously asked question.

Best practice is to be open and transparent and explain changed circumstances. It may be worthwhile to work with an attorney for such response.

2-year Green Card Expired - Is it absolute necessary to File Form I-751 with Form I-90 to renew card or I-90 can be sufficient?

Yes you must file the I751 regardless of previous filings. Unsure why you filed I90 for renewal if you have 2 year GC.

I received a Your benefit request has been accepted and is under review.” From USCIS for my I-130. What does that mean ?

It means that the application has been received and is pending review.

Not if your marriage is in good faith.

It means you were assigned an alien registration number, which is nothing to be concerned about.

Can a conditional green card holder be a joint sponsor for marriage-based green card application?

Yes a conditional PR may act as cosignor.