Justice is a fan-made global mod.

2) Children of Urgash and undead are vulnerable to Holy, while children of Ascha, i.e., the rest, are vulnerable to Unholy. Orcs are again in the middle.

Has the undead been separated from Ascha for a long time?

3) Those who practice Dark magic are vulnerable to Holy, and those who practice Light magic are vulnerable to Unholy. Orcs are neither here nor there.

Humans also dabble in darkness, as do mages who do not favor any one magic, but equally learn all schools.

Of course, the elves of Tuídhana are dark and evil, but they still worship the daughter of Ascha and do not fit into point 2.

And why would Unholy suddenly cease to be associated with Ascha – Unholy is not the magic of Chaos, not the magic of Urgash, but the magic of Darkness, i.e., the magic of Malassa, the goddess of Darkness and daughter of Ascha.

As far as I can tell, demons cannot be vulnerable to both "words" in all respects.

Demons are born of Urgash, elemental dragons are born of Ascha, the elements are separated from the original magic by Ascha, and each of the elemental dragons was associated with one of the elements. Elemental magic (any) comes from Ascha; the magic of demons is called the magic that comes from Urgash (it is not clearly revealed in the lore what this is, it is only known that it is a modified original magic - as in the case of Ascha, Urgash is a dragon of original magic, they both changed it, but Ascha ordered it by separating the elements from it, and Urgash, let's say, went his own way).
We get: all elemental magic is associated with Ascha, and therefore not associated with demons, and demons should be vulnerable to it. Demons do not worship Malassa and are not her creations, and they should not have immunity to her magic.

Added 3 minutes ago

Dark elves are just elves, the same as light elves, only they practice Darkness, live underground, and worship not Silanna, but Malassa.

Quite the opposite! Read the lore.

Added 4 minutes ago
"In desperation, Tuídhana makes a deal with the faceless in exchange for the power to protect her people. Those who follow her are changed by the will of Malassa, the Dragon of Darkness. And from that day on, they are called dark elves..."