thank you for your response!

Based on your experience, how long does it take to get B-2 visa if apply from Cananda?

My concerns right now is even after we jump through all the hoops to get the passport and I-94 extended, we would still have problem to get another B-2 visa relatively soon, i.e. even if my parents go to Canada and appy US B-2 visa from there, the immigration may still reject them.  What are they going to do then?  Would it be possible to go on a tour to Mexico and apply to come back to US from there after the visa expires and get a visa from there?