The police clearance requirement is harder to predict than the medical. You know that if the medical is more than a year old then a new one will be required at the time the decision is made. There are a few people on the other forum you are on that have sponsored spouses through China so they might be able to tell you more about whether or not another police clearance was required at the time of an interview/decision being made. If you are able to avoid the interview, it's doubtful another police clearance will be required.
And just an FYI, the police clearance can't be more than 3 months old when submitted to Mississauga so don't confuse the country's expiry date with the timelines you need to follow with CIC.
Don't get too bent on any obstacles right now. The system is really confusing at first but the key seems to lie in being well prepared, anticipating and addressing issues at the outset, and keeping a calm head. You are asking good questions and learning a lot right now. Pretty soon you will be one of the experts and surprisingly enough, you might also see how some of this makes sense when you read other stories about just what some people will do to get to Canada. It doesn't make it any easier on you as, like me, you probably never considered or concerned yourself with immigration law until it applied to your own personal life. I've now decided to make it my career and help others navigate these muddy waters!!! As Canadians, MOST of our systems make sense and follow a logical format. We are used to transparency, timely communication, and reasonable timeframes. This one is different and the logic is hidden away and takes awhile to see. And even when you see it, it can frustrate the hell out of you. So be patient, read more, and ask more good questions. You and your wife will be fine. At the end of the day, it's all worth it.
