How could Thorin and co. journey all the way to Erebor without discussing how to deal with Smaug?

I think the implication was that the dwarves' original plan called for Gandalf to be at the Lonely Mountain to help defeat Smaug. Gandalf himself doesn't promise this, and once he leaves to deal with the Necromancer, he pretty much leaves them to their own devices:

"It is no use arguing. I have, as I told you, some pressing business away south; and I am already late through bothering with you people. We may meet again before all is over, and then again of course we may not." (Gandalf, Chapter 7: Queer Lodgings, The Hobbit)

Once they had gained access to the underground city in the Lonely Mountain, the dwarves decided to start by sending Bilbo in to reconnoiter, hoping that maybe Smaug had left or died. I'd guess that, had he brought them back information on Smaug, they would have then decided on what to do next: wait for Bilbo, send back to Laketown for reinforcements, or come up with some trick that would allow them to defeat Smaug unaided. They didn't anticipate that not only would Bilbo run into Smaug and wake him up, but that he would leave immediately for Laketown -- and his doom.

It isn't spelled out exactly, but it seems like Gandalf's plan was to recruit Beorn to come along in the role of dragon slayer. He and the dwarves executed the same kind of routine they used to bring in Bilbo, just Beorn wasn't the joining type. Not long after that, Gandalf was called away to deal with Dol Guldur and Thorin's group was left on their own. At that point the plan seemed to be to reconnoiter the situation and see what opportunities arose to reclaim heirlooms, mainly the Arkenstone. Killing Smaug was clearly beyond what they could hope to do.

Thorin's worldview is thoroughly medieval. Besides Gandalf, Thorin didn't talk about big plans to any of the rest of the company because he was the nobility and the rest were there to do whatever he told them to do. Gandalf was the only person in the group Thorin respected as anything close to equal (at the least he had earned some respect by his connection with Thrain and having delivered the map to Thorin).

Neither Thorin nor Gandalf is a point-of-view character, and they are the ones with the knowledge and the plans. The story is told from Bilbo's point of view, and he was regarded as hired help (so didn't rank as high as the retinue which was at least family); it seems unlikely that Bilbo would be included in planning discussions.

Faith was a powerful force in Tolkien's world. As he had faith in his own abilities and decisions, I believe he projected that faith into his characters. Gandalf had faith in his Hobbit friend and faith in the stalwart companions he thrust him into business with. More importantly, Gandalf had faith in his own abilities to skew providence and likely ensure the success of his mission... which was to ensure the continued safety of Middle Earth and not simply promote a band of dwarves. I believe Gandalf's private plan was to spur Smaug to action before Smaug became someone's pet or ally; and given that, the less said, the better.

I believe WWI seasoned Tolkien in ways few of us could comprehend, let alone appreciate. Beyond his personal navigation and avoidance of several nasty ends, I think he noted the valor of the common soldier and recognized that valor as an occasional source of fantastic and improbable outcomes. Sargeant Alvin Cullum York comes to mind. (I'm from the U.S., but I'm sure there were fine British examples as well.)

Also those dwarves drank a lot... "We don't need a plan to kill a wyrm Master Baggins! Hold my meade and watch this!"

They know there is no hope. Approaching a dragon is a suicide mission! But their world is one of myth and feats and, especially, fortune.

Fortune will either smile on them, or it will not. But they have nothing to lose, they are fed up with having no home and menial work, and they want to restore the family honour. It's treasure or bust. (Getting rid of the dragon is a bonus.)

There are two things to consider— why they even begin without a plan in the first place, and why, during the journey, the topic never seems to come up.

This is my interpretation:

• They are partly in denial about their impossible mission. Nobody wants to be reminded of the dragon elephant in the room.

  • Further to that, it makes it easier to ignore the problem if they assume Bilbo has it under control.
  • Also "we'll cross that bridge when we get there." If they dwell too much on the final problem they might lose heart and quit before they've started.

• They are partly throwing their fate into the hands of fortune. Either they were meant to retake the mountain or they weren't. If they were meant to then some good fortune will come their way that they can't predict. Just as the map landed in their lap, which happened after they decided to head to the Mountain.

  So far we have had no clear idea what to do. We thought of going East...

  The point being, the only way they could possibly succeed is if something unexpected happened—and they know it. They know it is virtually impossible to kill a dragon, so there is not a lot of point talking about it. They set out bravely with a purpose and it is up to fortune as to how it will end.

• It was hard to make a plan when they knew so little of what had happened at the Mountain—they didn't know if Smaug was alive until they got there:

  The dragon is still alive and in the halls under the Mountain then—or I imagine so from the smoke," said the hobbit.

  Bear in mind that the Dwarves hadn't even gone west of Rivendell for a long time. In the first chapter they discussed the basics of dragons, but coming up with a detailed plan depended on too many question marks.

• The races of middle-Earth don't really get on well. Bilbo was always a bit of "hired help" and not one of the family. The Dwarves had their reservations about sharing too much with him. They ought to have had more detailed discussions with him, but he wasn't a professional burglar and didn't ask them for the right information.

It seems a little absurd that old tales of dragons hadn't come up "around the campfire" on the way across, but actually they had and we didn't hear all of them. In Beorn's house:

Some of the verses were like this, but there were many more, and their singing went on for a long while

I think it makes perfect sense as to why they set out without a plan. But it doesn't seem like they actually discussed what could be done about Smaug along the way. So it's either a topic they avoided, or really a plot hole.

I first read The Hobbit as an adult. My interpretation was that there were three different views:

The dwarves trust the competence of a professional burglar selected by Gandalf. The burglar will need to see the configuration of Smaug's cave, especially the side door, before deciding anything.

Gandalf knows that Bilbo has reserves of courage and cleverness that he is not using, and will never use if he stays in his comfortable hole. The writer, elf-friend, and poet of Lord of the Rings is unrealized potential at risk of being lost. An adventure is just what Bilbo needs to bring out the best in him. Gandalf does not want the dwarves to find out how little burglary experience Bilbo has, and does not want Bilbo to find out how much the dwarves are depending on him, until Bilbo is trapped and has to act.

Bilbo is totally confused, but assumes the rest of the party, especially Gandalf, know what they are doing.

None of those views call for any planning discussion, and Gandalf wanted to prevent discussion. Instead, they need to concentrate on the immediate problems of getting to Smaug's cave.

Partly: it's explored in the first chapter then hand-waved away.

This "plot hole" is in fact the driver of the entire plot of The Hobbit: the reason why a hobbit is involved at all.

"That would be no good," said the wizard, "not without a mighty Warrior, even a Hero. I tried to find one; but warriors are busy fighting one another in distant lands, and in this neighbourhood heroes are scarce, or simply lot to be found. Swords in these parts are mostly blunt, and axes are used for trees, and shields as cradles or dish-covers; and dragons are comfortably far-off (and therefore legendary). That is why I settled on burglary-especially when I remembered the existence of a Side-door. And here is our little Bilbo Baggins, the burglar, the chosen and selected burglar. So now let's get on and make some plans. "

So Gandalf has given some thought to the idea of fighting the Dragon and has decided that, as you say, the company of 14 is not enough to defeat Smaug. Instead, he reasons that knowledge of the side-door and a small and crafty hobbit will be enough to get into the mountain, explore its secrets and hopefully escape with some treasure. Bilbo summarises it well:

"Well, I should say that you ought to go East and have a look round. After all there is the Side-door, and dragons must sleep sometimes, I suppose. If you sit on the doorstep long enough, I daresay you will think of something.

At this point, Thorin mentions the obvious problem:

"Before we go, I suppose you mean," said Thorin. "Aren't you the burglar? And isn't sitting on the door-step your job, not to speak of getting inside the door?

And then the conversation is derailed by talk of breakfast and not bought up again. So although it's clear that the company is not meant to fight Smaug, quite how the burglary is supposed to proceed and what it is hoped to accomplish is quietly hand-waved away.

It's worth mentioning that this kind of plot-stretch is not uncommon in children's books, which is what The Hobbit is supposed to be. In the modern lens, due to its connection with The Lord of the Rings, readers tend to treat it more seriously. But children are more credulous than adult readers, and will just enjoy the adventure without worrying about plot holes.