Unless that is your only grade, that is ridiculous. I remember having only a handful of courses where there were only a few grades that were assessed for an entire semester. But one singular assignment?!?! Additionally, you should report her to the department Dean. Stuttering is clearly within ADA. As long as you are an active stutterer, you qualify under the guidelines of the ADA. Stuttering is not just the act of stuttering. Unfortunately, the utter humiliation of being in front of a classroom and told to "dance" (ie: speak & stutter) is degrading and comes along with it. It is just a part of life sure. But not something you should be forced to do. While most stutterers go through life never invoking any ADA accommodation, this is an instance where you may want to. You have given every reasonable solution, even if you were to do so with a deduction, since it is not the original assignment. But to be presented with an all or nothing ultimatum is akin to asking a wheelchair bound student in a PE course to either run up a flight of steps or be flunked.