Confronting Abeloth with light and dark was rather symbolical meaning. On practice she was killed by Krayt. All Luke did is restrained her, which allowed Krayt to deal mortal wound. In physical world Abeloth could be killed by anyone, even though she is still technically alive, it is still considered losing fight.
As of Son, he can be defeated with lightsaber too(this is how he died). He is not simply immune to lightsaber but rather uses Force to absorb blade energy or deflect with hands. If his effort is diverted to different Force application, lightsaber will wound him.
If you want to throw that GL quote in my face as evidence then you also need to consider that Luke by that time is 200% of Sidious. So it took a being who is 200% of Sidious just to restrain Abeloth.
Lol seriously? That's the example you use? With the death of his father The Son lost his powers and immortality. Did you not miss that whole Father stabbing himself with the dagger?
So sure a lightsaber can harm a powerless mortal son. Its pretty clear lightsabers can't hurt the Ones when Father grabbed Skywalker's lightsaber.
If lightsaber can't hurt them, why then father used Tutaminis on Anakin's lightsaber and Son used his hand to block Anakin's lightsaber?
Immortality is not a switch that activates with X amount of power.
Father was nearly killed by Son's lightning, it is proof enough that they can be hurt.
Abeloth is similar being to The Ones. Son gained power by bathing in Font of Power. Daughter - Pool of Knowledge. Abeloth is above either of them because she bathed in both. Yet, she could be wounded with lightsaber, explosions and Force attacks.
He didn't. He grabbed Anakin's lightsaber then pushed it back into its hilt.
1. Yes it is. *See Vitiate, Plagueis, and Sidious who all sought immortality though more power in the force.
2. The Father was knocking on death's door by that point anyway, so this proves nothing.
1. Luke himself states pretty blatantly that they didn't even kill Abeloth and only the dagger can truly kill her, which is why the Order went searching for the Dagger of Mortis.
2. Abeloth is not like the Ones. She started as a mortal being 100,000 BBY. The Ones are immortals who have been around for over 1,000,000 years. +Its stated that the power she gained twisted her far more than either the Son or the Daughter.