This is an interesting point. Radio buttons are kind of weird in terms of their initial position because unlike with checkboxes, there's no way to get back to the "empty" state where none is selected. It took me a bit to wrap my head around what you described, but the more I think about it, the more it seems like a "slider" with clearly marked stopping positions. If it's too confusing for people to have some sort of "default" option, maybe having a different color for the part of the slider that's moved based on whether it's in a valid position could help; at the top of the slider, it could be one color indicating "must be moved", and then when you slide and drop it at one of the marked options (or click the option; it could move automatically to the position like a scrollbar), it changes color to show it's valid.