J1 can be transferred. Or you can get H1b, either way is fine. J1 hold has 1 month grace period usually.
Only Chinese Embassy asks for a letter from your boss or your employer showing that they will sponse your H1b after J1. If your have a offer letter from HR, you can use that intead of your boss's letter. That's the way I did and on problem at all. DOS only needs your statement. Read waiver application package carefully, you will figure that out.