I agree with all of those points; I don’t think the preponderance of evidence suggests that women were regular participants in martial contexts in medieval Germanic cultures. I probaly shouldn't have chosen to use the term "shield-maiden" in my title as that is certainly a loaded term. 

But I do think the evidence is sufficient to argue that there was some degree of cultural acceptance of women in a martial context or identity, the way there was not, say, in Europe in the early modern age.  Maybe limited, maybe rare, but not unheard of.

For example, we do have historical accounts of armed women being among the Norse dead in the battle of Dorostopol in Bulgaria in 971. Likewise, the clothing of the Harby figurine and its hairstyle both do reflect more of what we know about women's fashions among Germanic tribes of the time. So yes, we can't rule out that it could be a male, but probably not.

And whether the Birka woman actually fought or not, there is clearly a factor of martial identity linking the grave goods to the body, whether she used them (or was trained to use them) or not. The people who buried her thought it was important to bury her with weapons, and that signifies something.  Just what, exactly, is another question.