Regarding L2 extension

[myname0]

My L1A extension I-129 was applied in Aug-2016 along with I-539 for dependents. It was then upgraded to premium in Nov-2016. I subsequently got the approval for my case (I-129) in Dec-2016. However for my dependents its still under processing. I have 2 questions.

1] If my dependents extension is not approved as of 240 days from I-94 expiry, do they have to leave US by then? Or can they continue to stay as long as my extended I-94/petition is valid, since my extension is already approved and their extension was applied before their I-94 expiry?

2] As per the latest processing time update dated 28-Feb-2017 on USCIS site, they are processing I-539 applications for L2 extensions applied as of date 2-Aug-2016. By when can I realistically expect an update on this case? This case was filed on 29-Aug-2016.