The kind folks of Intuit support helped me untangle the issue.
It is possible to enter any date for the end of fiscal year, if you edit the forms directly. Click on "Forms" in the upper left corner and find the fiscal year end date that needs to be edited, and edit it in place.
I still think that TurboTax's automated guide is wrong to not offer December as a possible end of fiscal year, but for some reason I could not find the fiscal year option choice anymore in my in-progress tax return.
I hope the above will allow you to work around the same issue, until turbotax is fixed.
