For ppl who are still stuck in either Canada or Mexico waiting for an interview you have the option of using AVR (automatic revalidation) even if your visa has expired.
I used the AVR to enter US at Newark Airport today evening from Mexico. All I had to do was show my I797 to the officer at immigration control.
A bit about my sitiation-
I had originally scheduled an appointment at the US consulate in Merida for June 18(DS 160 submitted on May 23).However one day before the appointment (and after landing in mexico) I got an email from the consulate asking me to reschedule it after June 27th due to the visa interruptions that everyone is well aware of. But the earliest I was able to get an appointment was only for July 7th.
I wasnt very keen to wait until July 7 and hence decided to use the AVR route after consulting with my lawyer.
Both the immigration officer at Newark and the United Airlines rep at Cancun seem to be aware of this rule and only checked for my I797.
So if anyone is stuck indefinitely in either Mexico and Canada and doesn't want to wait anxiously till the issue is resolved should look into using the AVR.
Of course this is assuming you did not appear for the interview. In that case plz consult with your lawyer.
Hope this helps.