I think you should seriously to consult a good immigration lawyer asap, instead asking questions here. The misrepresentation is a serious offense in Immigration Law. A simple letter from landloard may help, but it is not necessary enough to overturn the decision. Usually CIC will not accept an excuse of making an honest mistake, because on the application it clearly stated that as an applicant, you are 100% responsible for all the information you filled in. You may stand a chance to overturn it, if you have concrete evidences, such as your tax return to show that you filed tax as common law, after your landing. You need concrete evidences to connect all the dots together. Otherwise, you are expecting a certain refusal from CIC, which means they will probably bar your wife 5 years for misrepresentation, and you will also risk to have your PR revoked, should CIC launch an investigation against you.
If you get disapproved, and the disapproval is not on the ground of misrepresentation, your next application should be spousal application, not common law, since you were married.
And "can we declare we are common law on 2014 while in fact we are not but due to this is the reason of the disapproval are we force to declare that date(2014) as the start of common law?" This is exact the reason that CIC is suspecting you made the misrepresentation. Never Lie on your application.