According to Baxter Stockman after he was turned back into a human, he preferred being a mutant fly, as he was much more respected and also due to the new abilities and skills he got alongside with the mutation. He even pointed out that it was the reason he didn't continued attempting to create Retro-Mutagen for himself.
Both of the fires were caused by Oroku Saki himself (Saki accidentally knocked off the candles inside of the dojo, while Super Shredder was throwing Molotov Cocktails everywhere).
Another thing is that Hamato Yoshi was present during both showdowns. While he was a human and alive in the first showdown, he manifested himself as a ghost before Leonardo delivered the final blow to The Shredder.
Not only that but also, there is a major death that occurs during both showdowns that brings upon its conclusion (Tang Shen's death in the first showdown and Super Shredder's death in the final one).
And before The Shredder can deliver the final blow to Leonardo, something occurs that concludes the fight. Chris Bradford and Xever Montes resurfacing as mutants, which served as a distraction for the Turtles. In the final showdown whatsoever, Leonardo was able to kill The Shredder before he delivered the final blow to the turtle.
Another thing to point out is the outcome of both showdowns. While the first showdown had the Turtles retreating and indicating the initiation of a new threat to their lives, the final showdown had the Turtles be victorious and bring a conclusion to such threat.
The second reference is made by Tiger Claw when one of his interchangeable weapons for his mechanical hand that showed resemblance to Han's, the main antagonist of the film.
When Leonardo was pulling Rocksteady by his ears and exclaims that he has sensitive ears, this is a reference to the 1987 Teenage Mutant Ninja Turtles when Rocksteady exclaims to Shredder that his ears were sensitive as well.
In the climax of the final battle, Leonardo cuts off Super Shredder's head with his Katana Swords to finally kill him, which is a reference to the Mirage comic issue #21, where Leo decapitated Shredder as well.
After Bebop and Rocksteady were going to be attacked by their own traps inside of the dojo, Bebop was trying to climb on top of a fallen wrecking ball while Rocksteady stood on his left side. After Donatello winds up the traps, Bebop is standing on Rocksteady's right side and the wrecking ball is no longer seen.
When Tiger Claw launched his robotic hand at April O'Neil, there is a shot in which it is seen that the whole mechanical hand was shot at her when Tiger Claw had the wrist piece of the hand around his arm still.
In one of the shots during the scene when Super Shredder is lifting up Leonardo in the air, one of Super Shredder's knuckles has a green vein that he does not include in his design.
The title of this episode was confirmed months before it aired alongside with Requiem by Gwendoline Yeo on her Twitter account, when she posted confirming in which future episodes Shinigami would be in.
This episode aired on December 30th in South Korea.
There was a deleted scene in the episode of the assassination of Super Shredder, in which a line can be seen on his neck after Leonardo strikes him with his Katana Sword. Also blood can be seen flying from his neck, which was censored due to it being graphic for kids. The full image was revealed by Ciro Nieli on his Instagram account, but in the final episode the blood can still be seen dripping from the sword.