Yes. Of course it is permissible. It is your device. The wipe must have completed before arriving at the counter.

I wouldn't assume that to be the case. It's illegal under federal law to destroy evidence of a crime. Just what the government needs to do to show that you've destroyed evidence of a crime and not just the sexting you did with your girlfriend is a pretty murky area of law, from what I can tell.

I would not present a phone to customs that had clearly just been wiped.