Then all you have to do is show that you provided the DMV with your address change, before they sent you that registered letter. The longer, the better (what was it, at least two years?).
IF your current license shows the emission date, you're all set. Of course you should check if Washington gets your address change, when you surrendered your WA license to Idaho (this'll involve asking ID's DMV what their procedure is, and may be just like finding a needle in a haystack).
(You'd also have to get that in writing.)
Even better: if you could prove that you never were in WA state at the time of the alleged offense ("misdemeanor citation for driving on a suspended license in Washington state"), then that should take care of it too. Of course it's near impossible to do that. Your wife could make a good witness.
Sorry to hear about your troubles. I hope that the authorities will see it for what it is: an administrative SNAFU, and dismiss it all.:shrug: