Stay extension for B2 visa holder questions


I files stay extension(I-539 and I-539A) for my parents and it is still in process, got receipt number couple of days back though. I have couple of questions as below.

  1. Were my parents allowed to stay in US till they get a decision on the filed extension even after 6months of stay?
  2. Can they postpone flight tickets if they don’t get response from USCIS within 1 week of their planned initial flight to after requested 6 months stay legally?
  3. If they get denial of stay extension, how many days do they have to leave US after 6 months of initial stay.

Thanks in advance