is it legal for the lawyer to change my husbands letter without permission? wouldnt that be considered "doctoring" it?

and what about the fact that they withheld the prescription information?

The letter was not part of the decision to plead guilty, it was prepared for sentencing. If you are looking to present the evidence to the judge to seek reconsideration of the sentence imposed, that is certainly possible.

If you have the money to pursue an ineffective assistance of counsel claim and attempt to overturn the guilty plea so that your husband can go to trial, you can attempt it. There is a very high standard for reversing a plea, but if you have the money you may want to consult with an experienced attorney and give it a shot.