Was Crocodile stronger at Marineford? Or was he holding back in Alabasta?

 During the Alabasta arc, Crocodile displayed a level of power that was initially considered overwhelming by the Straw Hat Pirates. He possessed the Logia-type Devil Fruit called the Suna Suna no Mi (Sand-Sand Fruit), which granted him the ability to control and transform into sand. He had a reputation as a Shichibukai and controlled the desert kingdom of Alabasta from the shadows. His strength was showcased through his battles with Luffy and others. At Marineford, Crocodile was present as part of the war that took place at Marine Headquarters. While he did participate in the battle, he didn't display the same level of dominance as some other powerful characters present. This has led fans to speculate that he might not have been as strong as initially portrayed in Alabasta. It's important to note that power scaling and character abilities can be subject to interpretation and development by the author. Oda often keeps details deliberately open-ended to keep the story intriguing.

Could the Ring of Power have given the Witch King greater power and made him invincible?

 In J.R.R. Tolkien's The Lord of the Rings, the Ring of Power is a magical artifact that was created by the Dark Lord Sauron to enable him to dominate and enslave the inhabitants of Middle-earth. The Ring is able to corrupt those who wield it, and it grants them great power and influence, but at a cost.

In the story, the Ring of Power is sought by various characters, including the Witch-king of Angmar, one of Sauron's most powerful and feared servants. The Witch-king is a formidable opponent, with great strength and magical abilities, but it is not clear whether the Ring would have made him invincible.

While the Ring is able to enhance the power of those who wield it, it also has the ability to corrupt and enslave them, ultimately leading them to their own downfall. In the case of the Witch-king, it is possible that the Ring could have given him even greater power, but it is also possible that it could have led to his eventual defeat, as it did for other characters who sought to wield it.

Overall, the Ring of Power is a dangerous and seductive artifact that grants great power to those who wield it, but at a cost. It is not clear whether the Ring would have made the Witch-king invincible, as the outcome of such a scenario is purely speculative and is not addressed in the story.