Thank you.
It would seem to me that Radagast+normal female bear would not be an impossibility, but I rather think Tolkien would have balked at it. It feels to me more "mythological" for the mother to have been a human woman, as I can recall at least one Greek myth in which Zeus went to a mortal woman (Europa, I believe) in the form of a bull (though I don't believe they had relations while he was still in that form. Maybe they did. Will have to look it up). I believe there are other myths about various gods coming to women as one creature or another, with resulting unusual children. It just felt like a more logical connection (though how much logic can be applied to myth is certainly subject to debate
).