Does my employer have the right to insist I sign a release to allow them to talk with my doctor?

I had surgery in June and took one week off of work using some of my paid illness days available. I had made arrangements with my manager to work from home for the duration of my recovery. Due to some complications from my surgery I am still not able to physically make it back in the office and continue to work from home. My manager has denied any issues with the quality of my work while working from home. I have provided my employer with a request for accommodation form completed by my doctor, explaining my surgery and why I need to continue to work from home. My HR rep called me and said that I did not sign the portion of the form that allows them to talk directly with my doctor. I explained that the form reflects that by law I do not have to sign it and I do not want to sign it and that my doctor provided more than enough information explaining why I still need the accommodation to work from home and he'll reassess me in 3 weeks at my next appointment. The HR rep threatened me that she will go to their lawyers and tell them I won't sign the form to allow them to talk to my doctor and that there are other ways I can make it into the office.. I never filed a claim under FMLA because I did not need an actual "leave". Can my employer harass me into signing the medical release so that they can talk with my doctor directly? Can they deny me reasonable accommodation when it is not an undue hardship on my employer? Many employees with my company normally work from home and it is not due to a disability, it is just an approved situation. Many work out of state and work from home. A fellow employee works out of her home in another state on a full time basis.I am afraid they will fire me because I won't sign the additional release. Also, I previously did file a disability for a different medical condition in which I was unable to work at all and the HR rep mentioned this disability claim. I explained that I am not on disability leave and never requested disability leave and this is all related to my surgery in June and the complications that came up after the surgery. Can my employer deny me the reasonable accommodation ? I was told I can be fired by the HR rep for not letting them talk with my doctor. Is this a violation under the ADA?