Why wasn't Thorin aware about the state of Mirkwood and the Old Forest Road?

Question

In The Hobbit, in the "Queer Lodgings" chapter, Beorn explains to the company the dangers that they might face through Mirkwood:

“But your way through Mirkwood is dark, dangerous and difficult,” he said...

And the dwarves seem like they had no idea about it, or at least they didn't seem to consider crossing Mirkwood that dangerous:

But their spirits sank at his grave words, and they all felt that the adventure was far more dangerous than they had thought

Wasn't Mirkwood in this dark and dangerous state when the dwarves were still in Erebor and when they fled Erebor, since Sauron was in Dol Guldur during that time? Is the more detailed history of Mirkwood something that Tolkien only considered afterwards (when writing The Lord of the Rings)? (I guess a possible explanation is that Mirkwood was in a slightly better state when they left Erebor, since Sauron was gradually getting stronger.)

Is there any information available regarding how the Dwarves fled south from Erebor and arrived in Dunland? (e.g. if they crossed Mirkwood via the Old Forest Road etc) Because the company's original plan was to cross via the Old Forest Road:

By his advice they were no longer making for the main forest-road to the south of his land

Was the Old Forest Road known to be "open" when the Dwarves fled Erebor? Wouldn't at least Gandalf, who had visited Mirkwood more recently be aware of the state of the road (and Mirkwood), which according to Beorn:

Beorn had warned them that that way was now often used by the goblins, while the forest-road itself, he had heard, was overgrown and disused at the eastern end and led to impassable marshes where the paths had long been lost

Answer 1

Thorin and Company are basically a pretty feckless bunch, at least initially. They don't have swords until they pick up some from the trolls' treasure. They have the idea of stealing back Smaug's treasure one a piece or two at a time, which Bilbo correctly points out would take lifetimes. Not having scouted out the path they must take back to Erebor is merely part of their general lack of preparedness.

Gandalf mocks Thorin for his lack of preparation in this regard.

“We thought of going East, as quiet and careful as we could, as far as the Long Lake. After that the trouble would begin—.”

“A long time before that, if I know anything about the roads East,” interrupted Gandalf.

Moreover, not knowing about the miserable condition of the Mirkwood road is merely one of the worst examples of Thorin and Company's ignorance of the conditions they are heading into. Equally bad is that they don't know about the goblins raiding passersby through the Misty Mountains. Without Gandalf to save the dwarves, they never would have made it out of Eriador, having been caught by the trolls the first night they had to camp in the open, rather than stay at an inn.

So the answer is simply that the dwarves were absurdly underprepared. For the first half of their trip, they are heavily reliant on Gandalf, who looks after them until Bilbo is ready to take over as the expedition's protector and unofficial leader. Gandalf is more knowledgeable, but he never makes entirely clear how much he knows about how difficult the trip will be.

In any case, Mirkwood was presumably not as nasty a place when the dwarves fled westward from the Lonely Mountain. Indeed, it seems that the situation in the forest is getting rapidly worse at the time The Hobbit takes place. (This may actually be why the White Council had finally decided to drive Sauron out of Dul Guldur at that point in time.) In particular, it seems that the Forest Road was passable until relatively recently before the Quest of Erebor. However, there does not seem to be any specific information about whether Thorin and the others had passed along it, although I suspect they had not, given their ignorance of the terrain in The Hobbit.