Wouldn't be surprised if the exam gets pushed back. I don't think they'd cancel it, but they might delay it a month or two. Then again, with how hectic everything is, they may cancel it. Also, I would take anything that China says with a grain of salt. You really think they have fewer cases of COVID than the US and Italy? No way, no how.
I think averages will be the same. Any extra time you have now will be lost when pre-clinic/clinic resumes, so it all averages out. I'm assuming most schools are going to have their students do a bunch of catch up work.
This is N=1 but the dean of Penn's medical school said that in Philly, peak cases of Corona won't be seen until May. After hitting the peak, who knows how long it would take to reach manageable numbers and begin to open up everything again. Keep in mind the NBME is in Philly