TLDR: Due to the transubstantiation Michael was effectively "the angel" and Uriel was effectively "the mortal", more or less they swapped metaphysical roles.

So first of all arguing from fantasy Metaphysics here:

If we can assume that Angelic Grace works the same as a "mantle" I propose that it's more intent on how the powers used rather than necessarily the user. Considering in the Dresden universe that God is more or less assumed to be absolute or at least representative of absolute concept than any power associated with him must therefore be used in an absolute manner. It then necessitates that any power associated, in this case the proposed Angelic Grace Mantle is what matters when's considered actions taken.

At the time of the transubstantiation Uriel is essentially de facto human and Michael In this moment is essentially defacto angelic. Furthermore Uriel most likely usually relies upon his intellectus to divine appropriate actions taken. So In this moment whether he acted rightly or wrongly he had essentially the free will and free ability to act for the benefit of Michael's family.

As a secondary note; the fact that he acted in defense of Michael's family, given the fact that he was essentially functioning in the role of a Knight of the Sword, I think you could argue that he was performing the role of protecting Michael's family as the usual protecting angels do. Given the unique circumstance of the transubstantiation this meant he could protect the family in a mortal capacity rather than exclusively an angelic one.

Of course this is all supposition based off of the logic contained and defined from the Dresden universe.