Gandalf says, "I have no memory of this place," not because he'd never been to Moria, but because the vast, dark, and confusing mines disoriented him, and he'd previously traveled East-to-West, not encountering that specific fork in the path, plus his powers as a Maiar were limited in his Gandalf form, making precise recall difficult. It was a moment of being genuinely lost in the labyrinthine tunnels, requiring time to orient himself, rather than a complete memory wipe, leading him to focus on the danger ahead (the Balrog) rather than ancient details.
He had been through Moria (more than once), but if memory serves he had only gone East to West, never West to East. So when he came to this place before, he may not have realized it was a fork in the road as he was coming out if the fork, not into the fork.
The west gate of Moria was a secret that was lost to the dwarves long ago (at least it seemed that way in the movies, in the books that entrance was still in their records but the opening words were lost/forgotten), as non of the dwarves alive would have known of its existence, or the password, which even Gandalf has ...
In the books, Legolas actually screamed in terror when he realized that a Balrog was coming. It was the Balrogs that rounded up the elves for Morgoth, to be corrupted into orcs.
Gandalf's fate after meeting the balrog defies our experience. He is said to have died, went to Valinor, and was resent to Middle earth as The White to continue the task. Tolkien obviously felt that such an experience justifies some lack of memory.
Tolkien's description of Gollum conforms to a Catholic – and Thomistic – account of envy, which is a “sadness of the soul”; and it is Gollum's unbearable sadness and his unquenchable desire for the one Ring that marks his character.
It's possible that Gollum's prolonged search for the ring allowed it to maintain a significant power over him, suspending his aging process. In contrast, Bilbo's exposure to the ring's influence waned after he willingly (mostly) relinquished it, allowing his aging to resume more noticeably.
Tolkien's Legolas is extremely introverted. He just doesn't have much to say directly to anyone, perhaps because he's thousands of years old. His odd relationship with Gimli draws him out in a way the hobbits never do, and that's mostly after Frodo and Sam leave the Fellowship.
As a Maia, Gandalf was an angelic being in human form, in service to the Creator (Eru Ilúvatar) and the Creator's 'Secret Fire'. He took on the specific form of an old man as a sign of his humility. His role was to advise but never to attempt to match Sauron's strength.
Why was Sauron afraid of Aragorn potentially possessing the Ring? Gandalf explains it in the Return of the King (during the council after the battle of the Pelennor fields) that Sauron would never think somebody would want to destroy the One Ring.
It's not the years; it's the mileage. Gandalf isn't a Maia in the same way Sauron is. He was sent to middle earth as an old man, so his body is much more real than Sauron's “raiment”. He can't use his power to the same extent, he can't change his forms at will.
If Gandalf had stayed dead after his sacrifice in The Fellowship of the Ring, then his demise would be the saddest in the trilogy, but since he came back, the saddest single death scene of all three movies goes to Boromir's.
Instead, we have to consider that Sauron, the original owner, is not human, but has a quasi-divine status. Therefore ownership of the Ring is subject to Divine Law. With this precedent in mind, many of the ambiguities clear up nicely. After numerous transfers by violence and trickery, Frodo is the true owner.
Even at his fullest, Olorin/Gandalf was not the equal of Mairon/Sauron, not even close. Same goes for the Balrogs, they were described as lesser spirits while Sauron was described as a greater spirit amongst those that were corrupted by Melkor.
Frodo left Middle-earth due to lasting injuries from his journey, seeking healing in the Undying Lands. His decision was influenced by trauma and a desire for peace, similar to Tolkien's own experiences. In Rings of Power, Frodo is portrayed as a hero who found solace and closure in the Undying Lands.
Several commentators have seen Gandalf's passage through the Mines of Moria, dying to save his companions and returning as "Gandalf the White", as a symbol of the resurrection of Christ. Like Jesus who carried his cross for the sins of mankind, Frodo carried a burden of evil on behalf of the whole world.
Gandalf does not WANT to touch the ring. Just as Galadriel resists the temptation to take it when Frodo offers it to her willingly. And all this because "Power Corrupts". Sauron and the Ring are allegories for the corruption that inevitably arises when too much power is concentrated in one place.
But that's actually the answer: Eru Said So . The Undying Lands are a place for immortal beings (Eldar), and mortals are forbidden from entering.
Firstly, being there does not confer immortality, contrary to what the Númenóreans supposed. Secondly, those mortals like Frodo who are allowed to go there will eventually choose to die.
Well legolas acts like that cause he knows aragorn is a badass and better than him. Also he knows that aragorn is the last descendant of the true king of gondor so he respects him for that too. Knowing that he is one of the last hopes for the kingdoms of men.
One of the things I like most and am proud of in this world is that Viggo Mortensen was offered to play Aragorn again in The Hobbit trilogy, but he declined because he said Aragorn did not appear in the book.
In the books Bilbo hadn't even come close to catching up with his biological age until after the Ring was destroyed. (And since Gollum was destroyed with the Ring, there wasn't any chance for his age to catch up with him.)
A group of UCL medical students, led by Dr. Liz Sampson, concludes that Gollum was actually suffering from schizoid personality disorder.
Sauron held the One Ring for the longest time, around 1,840 years until it was cut from his hand by Isildur. Sméagol was in possession of the Ring for the second longest time, 478 years. Bilbo had the Ring for the next longest period, 60 years, which is more than three times the length of time that Frodo had it.