It’s amazing how people have selective amnesia. Yes, Luke did throw away his saber in ROTJ, but a few seconds before, he was about to kill Vader. And a few minutes before that, he’s prepared to “strike down” the Emperor, only to have his faithful apprentice intercept the attack.
I would call that a character arc. These things didn’t happen in a random order. Luke get’s goaded into using force, first by Palps, then by Vader to the point of almost killing Vader. Luke realizes he’s about to follow in his father footsteps, and composes himself, tossing away his lightsaber. He’s learned a valuable lesson, and passed the test. A test he then again fails in TLJ.
How did he fail that same test? Aren’t his actions just consistent with how he was portrayed in ROTJ? A person doesn’t stop making mistakes just because they make the right decision one time. As you pointed out, the Emperor goaded Luke into striking. Vader goaded Luke into striking. He seems to like striking and be easily goaded and then later comes to his senses. How then is going to Ben’s hut to confront him and freaking out about how far he has fallen and igniting his light saber out of character. It seems totally in character by the very example you have provided. That is his MO. React on instinct and then let his wisdom rein him in. You have just proven with your own examples that Luke’s actions in Ben’s hut are 100% consistent with ROTJ Luke.
I don’t agree. The idea of a character arc is, that people make a journey, usually learning from their mistakes, such that they don’t make that mistake again. In TESB Luke drew his weapon first when confronted with Vader, itching for a fight. In ROTJ Luke had to be goaded into a fight, and he resisted the Emperor for a long time, despite the real threat and suffering he faced. Luke learned from his experience in TESB, as he learned from his experience in ROTJ to finally become a Jedi. So, after ROTJ Luke should have grown beyond such mistakes in my view. The situation with the possible future of Ben Solo pales in comparison to Luke’s previous real experiences from my point of view, and so he should have been able to control himself. Let’s not forget Luke has had at least two decades to learn from his experiences before his fallout with young Ben, and to grow as a Jedi Master. Yet, TLJ seems to suggest the opposite happened. Luke regressed, and turned out to have become a far worse Jedi than Obi-Wan or Yoda, despite growing beyond their dogma in ROTJ.
When did we see Luke actually learn from his mistakes? We see him make similar mistakes over and over again and we never see him really grow past it. And why should the fall of his nephew not shake him to the core? You are building Luke up as a legend and not seeing the flaws he carried right to the end of ROTJ. When Luke threw down his saber and faced the Emperor he knew that he was likely to die. The Emperor didn’t goad him any further, he just tried to kill him. But there is no big huge change in Luke’s personality so him making a similar mistake due to the horror of seeing how far his nephew had fallen already could be shock enough for him to act on instinct over intelligence for a moment. I don’t know why that is so hard to believe of his character. His arc was to redeem his father, not to be the perfect Jedi.