8老師, question for I-131 again, please help

 my parents's I-94 showed the expiration date is in Oct., but B2 visa will expire in Feb, 2015.
Can they still use B2 to leave and return to USA without I-131 application during their green cards process?

I am worrying if filing I-131 will affect their green card approval because they just came to USA for two months.

if I put August as the Date in the form I-131  for 3 to 4 months, will UCSIC deny the application due to a short time stay in USA.

Thanks.