[Sauron the White]

1. Smaug could not land in Laketown without the bridge.

There is not one iota of proof offered to support that claim. Not one. Just the opposite. We know that boats were loaded and unloaded so there had to have been dock space for that purpose. Smaug - if he wanted to land - could have landed there at the least.

2. Because Smaug could not land, his vulnerable underbelly was exposed. He was, in fact, killed because of this.

The deliberate destruction of the bridge to achieve this goal would only make sense if they first knew about the structural flaw on Smaugs belly. But they clearly did not know of it. Thus, the destruction of the bridge to aid in the downing of the dragon because of his exposed belly was not on or in anyones mind. So if this is your reason for the townspeople destroying the bridge, it clearly could not have been something they were aware of.

3. Smaug feared the water, and the text itself says, in as many words, that he "was foiled", and describes him as originally making for the bridges in his attack.

As long as Smuag had the ability to fly and retreat when tired it is irrelevent as to what he feared. I fear heights but as long as that airplane stays up its no big deal. Smaug never intended to go into the water or on the land for that matter. His attack was strictly aerial from start to finish.

What was he "foiled" in? In destroying the bridge himself? Some of his fun was spoiled? Or maybe Smaug intended to wipe out scores of fleeing townspeople easily as they bunched up closely running across that bridge to possible freedom from his destruction? Again, his fun and intentions were foiled and spoiled. Or in landing upon it and walking over... for what conceivable purpose?

Please quote the section of the text which states that Smaug feared the water. He did not go into it because of the vapor that would rise thus blinding him to the escaping people and it would quench him putting out his fires. But it would hardly harm him.

4. The bridge did not offer a viable escape-route anyway, because the water itself offered more protection from a fire-breathing dragon than did the land.

ANd what supports that belief? A boat on the water was subject to the same aerial attack that those on the ground, or on the bridge, or those who would have reached mainland would have been subject to. There is no difference.
In fact JRRT tells us that Smaug cared not if they went into the boats because the dragon greatly enjoyed the sport of hunting them.

"They could all get into the boats for all he cared. There he could have fine sport hunting them, or they could stop till they starved. "

So JRRT himself says that Smaug intended to hunt them while they were in the boats on the water.

And JRRT himself said from the time the dragon was spotted to the time he arrived was brief.

"Still they had little time".

Given the "little time" they had for Smaug to close that distance, just how did they manage to quickly get organized, grab the proper tools and supplies, divide into divisions of labor, and take down and destroy that bridge? Was this like the River Kwai where the bridge was wired with explosives? I think not. Just how did they destroy that bridge so quickly in the few moments it took Smaug to arrive and attack the town?