Double check with WoJian, pls

I have a very urgent problem.
I have OPT EAD card, valid from Dec 1st,03.
An US company who is interviewing me, want fly me to HK and Shanghai next week to well understand their business in Asia.
My question is:
1. Is that sure I can come back to US? (This US company just interviewing me, did not get me any offer yet.)
2. How long I have to wait in Shanghai for a visa to come back US.
3. Can I fly to HK first then fly to Shanghai after?

Many many thanks to who ever can give me some answer

Wojian, I just got a friend answer me: NO.
But if I cannot make the trip, that will seriously effect that the company who will give the offer, I want the job, what I can do? Thanks for answer.