Pilgrim,
“It is BOTH that God's sovereign decree could not be thwarted AND the Lord Christ in His humanity had the ability to sin”, If He had the ability to sin, was there ever a chance in His humanity to sin? If so there are only two possible outcomes:
A.Humanity is without Christ vicarious substitutionary death in our stead
B.Humanity has Christ’s vicarious substitutionary death in our stead.

If there was no chance, then Impeccability is true.
Geomic

Death dog (horse) must still have breath.