Is the son's leaving the home he built, due to his father's rejection of his religious commitment (his wife's niqab) and his threat to expel him, considered a severing of ties with his parents, despite his continued financial support for them?

If the father commands disobedience to the Sharia, such as the wife abandoning the niqab, then obedience to him in that is not obligatory. Nor is the son considered disobedient if he leaves his father's house and lives in another house; rather, that might be better, provided he continues to honor his father and does not abandon him. And among the greatest acts of righteousness is to invite him and his brothers to Allah, to advise them, and to enjoin what is right and forbid what is wrong.