Do we need InfoPass to get extension for 10 renewal

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DP had his green card up for renewal after 10 years in March 2017. But, in Nov 2016, he applied for extension, and went for biometrics and they gave him six months extension until Sept 2017.  His priority date is 11/19/2016. Called up USCIS hotline and the agent said that the current priority date for 10 year renewal is 11/12/2016, and it might take about 2 months more. She suggested we make an appointment with USCIS and get an extension until the GC comes in the mail.

Questions:

Do we have to get an extension or can we just wait for the GC to arrive?

If yes, then is going for an extension a common route to take?

Any help/guidance appreciated.

Thank you.

 

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Any suggestions? Looking to get any answers on this.