So, the big battle. We come within a hair's breadth of a battle between dwarves on the one side and elves/men on the other, before Gandalf intervenes to say goblins/wargs are on the way. He throws in a mention of who is leading, and a reminder to the dwarves of why they don't like those goblins (a previous war). The question arises as to what Gandalf would have done if, say, the goblins were arriving a day later. I have to assume that he would have said something before the actual battle began between the dwarves, elves, and men.
It's also the only time we get a hint that Gandalf might be able to appear in a spot magically, when he shows up in between the armies in a cloud of smoke. I'm guessing, though, based on all the other times when he does not seem to be popping about magically, that this is just a way of saying "appeared" as in "was noticed to be", like you or I might appear somewhere. The smoke effects are probably his way of trying to make as theatrical a proclamation as possible, since he is after all trying to dissuade two armies, who are all set on fighting, to instead join forces.
One parallel that just occurred to me is the ancient (Second Age) history in which the dwarves of Khazad-Dum, who had let Galadriel and company travel through not long before, were unwilling to open their doors for the elves of Eregion when they were fleeing from the forces of Sauron. It led to a lot of bad feelings between the (surviving) elves and dwarves, and there could have been the potential for something like that to happen here, if Dain had managed to push his way through into the Lonely Mountain. Gandalf might have let it come down the the eleventh hour simply because he didn't trust the different sides to actually act together if they had any time to come up with schemes to do otherwise.
As for strategy, I have to wonder if it might not all have gone much better if they had simply skedaddled inside the Lonely Mountain, and mowed down the goblins and wargs as they tried to force their way in. But, this would have been tricky to arrange, since once inside the Lonely Mountain the men and elves could hardly have been kept away from the treasure hoard, nor evicted again, which is probably more trust than it was possible to build prior to the battle. Still, halfway through the battle it seems like the obvious thing to arrange an orderly, faces-to-the-enemy retreat inside the Lonely Mountain. I suppose that would leave the folk of Laketown outside the mountain, and after realizing they could not defeat their enemy with main force the goblins might have noticed this fact.
Most likely, though, as Tolkien indicates, the main problem is that there is no time to make plans of any complexity. It does indicate, though, just how much the elves, men, and dwarves will all lose due to their inability to sort things out ahead of time. If they had made friends ahead of time, they could have faced the goblins and wargs from an inpenetrable fortress, and lost few of their own doing it.
Bilbo, just after seeing the eagles coming, is hit by a rock and knocked out. This is another of several "it's not your great-great-great-grandfather's epic tale of battle" moments. Bilbo is mentioned to have taken great delight later in life in telling tales of having been at the Battle of the Five Armies, even though he did almost nothing in it. Bard, presumably, did lots, but we don't hear anything about it. There is a more conventional tale of the dragon-slaying hero here, but we are hearing Bilbo's tale instead. Given that JRRT was quite familiar with Beowulf and many other old sagas, this seems like an intentional diversion away from the path of traditional tales.
Chapter 18