Last June Wells Fargo (NYSE:WFC) was the one US bank to escape a Moody's rating cut a move largely attributed to the bank's exposure to Europe and investment banking operations.   Since then the stock's movement has been stable giving investors reason to pay a premium for conservative commercial and consumer banking practices and a lack of emphasis on riskier ventures like credit derivatives. But ex-regulators and industry experts have questioned whether Wells Fargo is overexposed to the housing market.  In Q3 of 2012 the bank originated a staggering $139 billion in residential mortgages -- 29% of those in ...