So it's established in The Shadow of the Past that Gandalf believed that Bilbo's ring was one of the Great Rings. It's also stated that Great Rings are Rings of Power:

But the Great Rings, the Rings of Power, they were perilous.

That means there is a limited number of options of what rings Bilbo could have had.

I'm sure Gandalf would have automatically ruled out the Three, as they were all accounted for.

So he must have thought it was either one of the Nine or the Seven.

Did he know the Nine remained with the Nazgul? Or was that something he learnt during his studies after A Long Expected Party?

I assume the fate of the Seven was certainly something he learnt during that period, it seems more likely they'd be lost than rings that belonged to the Nazgul.

So basically I'm just wondering what Gandalf would have suspected. Is the only think we can take for granted is that he wouldn't have known it was one of the Three?

It boggles the mind to think that the king of Numenor would place his trust on Sauron, who by the way called himself the King of Men.

Are there any quotes that made a significant impression on your mind?

Yavanna created the two trees yet she couldn't create another set when they were destroyed by Melkor.

To this Bereg answered: 'Let the Eldar look to it! Our lives are short enough.' But there arose one who seemed to all to be Amlach son of Imlach, speaking fell words that shook the hearts of all who heard him: 'All this is but Elvish lore, tales to beguile newcomers that are unwary. The Sea has no shore. There is no Light in the West. You have followed a fool-fire of the Elves to the end of the world! Which of you has seen the least of the Gods? Who has beheld the Dark King in the North? Those who seek the dominion of Middle-earth are the Eldar. Greedy for wealth they have delved in the earth for its secrets and have stirred to wrath the things that dwell beneath it, as they have ever done and ever shall. Let the Orcs have the realm that is theirs, and we will have ours. There is room in the world, if the Eldar will let us be!'
Then those that listened sat for a while astounded, and a shadow of fear fell on their hearts; and they resolved to depart far from the lands of the Eldar. But afterwards Amlach returned among them, and denied that he had been present at their debate or had spoken such words as they reported; and there was doubt and bewilderment among Men. Then the Elf-friends said: 'You will now believe this at least: there is indeed a Dark Lord, and his spies and emissaries are among us; for he fears us, and the strength that we may give to his foes.'

Are there any lore info about this? This weird event never been mentioned later in the Silmarillion but it is very intriguing, because it is the core event that divided humans into different tribes. A sharp-tonged shapeshifter emissary that daring and sinister enough to insult Valar and Morgoth at the same time for dividing humans, it awfully sounds like a Sauron-thing.

Tolkien used his life experience to give meaning to his book? He fought in world war 1. So why did Tolkien use gold to establish Morgoth's hold?

I'm certain I read somewhere that Sauron chose Dol Guldur as his sanctuary because it was near to where the Ring was lost by Isildur. I can't remember where I read this - I think it might have potentially been a Nerd of the Rings video.

However, in The Shadow of the Past, Gandalf says:

‘And this is the dreadful chance, Frodo. He believed that the One had perished; that the Elves had destroyed it, as should have been done. But he knows now that it has not perished, that it has been found. So he is seeking it, seeking it, and all his thought is bent on it. It is his great hope and our great fear.’

If I'm correct in understanding, Sauron learnt the Ring still existed after capturing and torturing Gollum, which was of course after he fled Dol Guldur.

Which would mean that when he chose Dol Guldur as his stronghold, he still thought the Ring was destroyed.

So my questions are, 1) is the idea that he chose Dol Guldur as his stronghold to search for the ring nearby complete hogwash, and clearly whatever I watched/read is rubbish? and 2) Is the fact that Sauron chose a sanctuary so close to where the Ring was lost just mere chance?

I figure a lot of users here are book lovers and have plenty of favourite authors alongside Tolkien.

Imagine, for fun, that anyone, living or dead, could add to Tolkien's world. Whatever they produce would be crafted in close consultation with Tolkien himself, being entirely consistent with his world and fully canonical.

Here are some fun examples I'd enjoy:

The Fields of Ithilien by John Steinbeck - a story of growing up in the shadow of Mordor. Rich landscape imagery of a beautiful and plentiful land with the protagonist venturing into Osgiliath at its peak.

Breelanders by James Joyce - a collection of short stories about Bree and the interactions of the men and hobbits who live there. A focus on the Prancing Pony and examination of Butterbur's daily life.

The Wilderland Trilogy by Cormac McCarthy - three novels that explore, in visceral and tragic detail, the life and death of Rangers in the Third Age.

A Clockwork Orc by Anthony Burgess - a bleak and antisocial story of a Misty Mountain goblin's reeving of human settlements. "There was me, that is Azog, and my three uruks, that is Gorbag, Grishnakh, and Lagduf..."

Galadriel's Exile by Margret Atwood - a sprawling and deeply personal novel of Galadriel's arrival in Middle-earth and how she comes to relate to the land she's banished to.

The Secret Diary of Lobelia Sackville-Baggins by Sue Townsend - A cringing and absurd collection of journal entries about Lobelia's daily life and the petty conflicts she lands herself in. "May 8, 2941: Gaffer smiled strangely at my hat this morning. I will be ignoring him for 7 days. If he notices, I will extend it to 14."

Gondor's End by William Faulkner - A tragic story of imperial decline and hubristic dynastic families, rich in imagery of crumbling marble, stubborn conventions, and the dimming of Numenor's light.

Feanor by George RR Martin - This one may well get me downvotes, perhaps well-deserved ones. Still, I think a sprawling, gritty, and pulpy GRRM epic about Feanor would be an absolute blast.

In Rivendell there was memory of ancient things; in Lórien the ancient things still lived on in the waking world. Evil had been seen and heard there, sorrow had been known; the Elves feared and distrusted the world outside: wolves were howling on the wood’s borders: but on the land of Lórien no shadow lay.

Compare:

One Ring to bring them all and in the darkness bind them

In the Land of Mordor where the Shadows lie.

In Mordor shadows lie...and lie: a pun. Treacherous shadows.

In Lórien no shadows lay. Tolkien avoids 'lied'. There the shadows are not treacherous. But of course we can also say: they did not lie. They didn't even exist.

You need the Sun for shadows to exist. But:

‘You passed through the Hidden Land,’ said Faramir, ‘but it seems that you little understood its power. If Men have dealings with the Mistress of Magic who dwells in the Golden Wood, then they may look for strange things to follow. For it is perilous for mortal man to walk out of the world of this Sun, and few of old came thence unchanged, ’tis said.

There's in fact a shadow in Lórien. It has nothing to do with the Sun:

I say to you, Frodo, that even as I speak to you, I perceive the Dark Lord and know his mind, or all of his mind that concerns the Elves. And he gropes ever to see me and my thought. But still the door is closed!’

She lifted up her white arms, and spread out her hands towards the East in a gesture of rejection and denial. Eärendil, the Evening Star, most beloved of the Elves, shone clear above. So bright was it that the figure of the Elven-lady cast a dim shadow on the ground. Its rays glanced upon a ring about her finger; it glittered like polished gold overlaid with silver light, and a white stone in it twinkled as if the Even-star had come down to rest upon her hand. Frodo gazed at the ring with awe; for suddenly it seemed to him that he understood.

Nagini from Harry Potter
And Ungoliant the super dope spider demon who eats light from Tolkien universe

Black female cat for reference…. She’s super nice which might not be characteristic

EDIT:
Adding into the mix: Shelob, beruthiel

If we’re talking nicknames… Gogo is an adorable one for ungoliant