My wife is US citizen and came to Canada 6 months ago. Her visitor status expires Jan 18, 2013. I have already applied for extension but she may not get it before we travel. Will she be able to return to Canada if she visits US for a week in march?
There's no black and white answer - even if she gets the extension. The decision of whether she is allowed into Canada or not will be up to the immigration officer your wife encounters when she re-enters Canada.