Should we apply for B1/B2 extension for 2nd time while the first extension application is pending or wait for decision letter?

Your case is way beyond normal processing. Make an Infopass appointment as soon as you can and get a status update.

I would make an infopass to determine if the case is approved. Otherwise, I recommend your mother leave before March 2nd.

You should timely submit a new I-539 with a copy of the I-797 receipt for the first I-539. My client had the exact same issue as you. She was admitted to the United States on a B2 Visa from June 18, 2014 until December 17, 2014. We timely filed an I-539 on December 15, 2014, requesting an extension of the tourist visa until June 15, 2015. As of June 2015, no decision was made on the I-539. On June 15, 2015, I timely filed a second I-539 with a copy of the receipt for the first I-539. On November 17, 2015 I received two approval notices in the mail, one for the first I-539 and one for the second I-539. Therefore, you should timely submit a second I-539 with a copy of the receipt for the first I-539.
One issue that you should consult an immigration lawyer about are the possible consequences of unlawful presence if the application for the first I-539 is denied.

The guidance and/or advice provided herein does not create an attorney-client relationship and is for general informative purposes only, One should only take action after retaining an immigration attorney.