I would say that "hassled" is not a harsh enough word for kidnapping. British men-of-war intercepted American ships and impressed American citizens to serve in their navy.
This was also only one of several motives for going to war. There were great fears on the frontier that the British were stirring up trouble with the Indians, although the reality was probably less galling (the Indians traded with the British and used British weapons and tools). Napoleon had also interfered with our ships and trade, but France had traditionally been our ally, and the Jeffersonian Republicans (the party in power) had no love lost for the British. Finally, as you correctly point out, Canada seemed to be an easy conquest, a "mere matter of marching" according to Jefferson.
2) "Granted, the U.S. did win some battles, but once Britain was freed from its involvement against France, defeat was inevitable."
You deliberately belittle major American successes in the Great Lakes region. Perry smashed the British fleet at Lake Erie, winning control of the important waterway. And General Harrisson, future president, recaptured Detroit and crushed a combined British/Indian force at the Battle of the Thames. It was here that the Indian leader and visionary Tecumseh fell.
When 1813 came to a close, the result was stalemate, not inevitable British victory. The American invasion of Canada had ended in failure, but the British gains in Michigan had also been thrown back. On the high seas, the British navy had imposed an effective blockade of the coast, but American privateers were tearing into the British merchant fleet.
Napoleon wasn't exiled to Elba until mid-1814. It was then that Britain geared up to crush the U.S. with three thrusts. An attack south from Canada along the Hudson river, with the goal of capturing New York City and separating an increasingly restless (and rebellious) New England from the rest of the country. A thrust into Chesapeake Bay designed to capture Washington, D.C. and occupy Baltimore (a major privateer base). And finally an assault against New Orleans, which would close the mouth of the Mississippi.
3) "In July of 1814, Moose Island and Eastport, Maine were captured and occupied by the British, who forced Americans there to pledge an oath of allegiance to King George."
All this is true, and there were many similar raids by the British blockading squadrons, but you are making mounds out of molehills. Although these raids contributed to the increasing unhappiness of New England, they were annoyances in terms of military impact.
4) "The following August, the British defeated U.S. forces at Maryland, and on August 24th burned down the Capitol Building and the White House."
Again true; this was the Chesapeake thrust of the British master plan. You neglect to mention that it all ended in failure. The Americans rallied and stopped the British army short of Baltimore. The British navy, after routing a motley group of American gunboats, then tried to pound Fort McHenry into submission and force Baltimore to surrender. As anyone who as ever heard the Star Spangled Banner knows, it didn't work, and the British were forced to retreat.
5) "Parts of New York were captured by Sir George Prevost the next month..."
Another glaring omission. Prevost lead the British attack down the Hudson River to New York, an attack that met with initial success (and a warm welcome from the locals). But at the Battle of Plattsburg, fought (ironically) on September 11, 1814, Prevost's supporting fleet was smashed by a smaller American force on Lake Champlain. In an era with no railroads and poor roads, the British depended on their fleet for supplies: no fleet meant no invasion. The second thrust of the British master plan also ended in failure.
6) "The British threatened to invade Boston next, at which point the U.S. finally agreed to sign a peace agreement."
I'm not sure where to begin on this one.
Peace parleys had begun as early as 1812. Russia, united with Britain against Napoleon, had no wish to see her ally fretter her strength away in what was essentially a Napoleonic sideshow. The attempt broke down, but the groundwork was laid for latter negotiations.
In 1814, British and American peace delegations met in Ghent, Belgium. The British foreign office, busy working on rewriting the map of Europe, sent it's second string to Ghent. The English negotiators were justly confidant: the American invasion of Canada had been repulsed, their blockade was strangling the Eastern seaboard, and the defeat of Napoleon had freed up plenty of manpower to implement their 3 pronged attack (New York, Baltimore, New Orleans). In light of these successes, they demanded control of the Great Lakes, an Indian buffer state, and a substantial chunk of Maine (where they intended to build a military road). The Americans flatly rejected these demands without even waiting for word from the State Department.
When two of the three British attacks were defeated (Plattsburg and Baltimore), the Foreign Office reconsidered its position. The Duke of Wellington, offered command in North America, wouldn't take the job unless Britain could guarantee control of the Great Lakes, which it couldn't. At sea, the American privateers were still eating into the British merchant fleet, which sparked British merchants and traders to call for peace. Britain was war-weary and debt burdened from her 20-year clash with France. Finally, European politics (the Congress of Vienna was ongoing) took precedence over the American war. The continent was in serious disarray from the French Revolution; stability needed to be restored and the balance of power worked out. The Foreign Office told it's second stringers to mellow their tone.
Things weren't going great on the American side of the hill either. New Englanders were meeting at the Hartford Convention, and were threatening to leave the Union. British sponsored Indian tribes controlled what would become Minnesota, Wisconsin, Illinois, and Michigan. The British blockade was more effective than ever, the economy was in ruins, and it wasn't clear if Jackson would win at New Orleans or not.
Washington had been destroyed, and parts of Maine were occupied. Rather than conquer Canada or right the wrongs of impressment (a practice that, as you correctly pointed out, had been discontinued) the American government, like their British counterpart, just wanted the war to end.
On December 24, 1814, the Treaty of Ghent was signed. Stalemate on the battlefield mirrored stalemate at the peace table: the only term in the treaty was that both sides agreed to stop shooting. No territory changed hands. No reparations were paid. The peace, like the war it ended, was a draw.
7) "In retrospect, it was obvious...that we lost the war, having failed in our expansionist goals, having lost ground to the "enemy," and having expended human lives and extraordinary resources for naught."
We failed in our expansionist goals, but we didn't lose any ground to the enemy. In point of fact, the British had also failed in their goals: no Indian buffer state and no control of the Great Lakes, although a future treaty would eventually get them the Maine land and their military road. We didn't beat the British, but they didn't beat us either: it was a draw.