"Considering?" Do you recall a specific citation that shows Seward attempting to provoke, warn, or otherwise suggest the American intervention between Canada and Great Britain?
A few.
"She [Canada] cannot refuse if you tender her annexation on just terms, with indemnity for the stuggle she may expect with Great Britain"- William H Seward, 31 January 1856
"Hitherto, in common with most of my countrymen, as I suppose I have thought Canada — or, to speak more properly, British America — a mere strip lying north of the United States, easily detachable from the parent State, but incapable of sustaining itself, and therefore ultimately, nay, right soon, to be taken into the Federal Union, without materially changing or affecting its condition or development. (Seward, Albany Journal, 1857)
In 1860 Seward had informed the Duke of Newcastle at a public function that as soon as he got into office (Seward being a candidate for the Republican party nomination) he would insult England.
"We must CHANGE THE QUESTION BEFORE THE PUBLIC FROM ONE UPON SLAVERY, OR ABOUT SLAVERY, for a question upon UNION OR DISUNION... FOR FOREIGN NATIONS, I would demand explanations from Spain and France, categorically, at once. I would seek explanations from Great Britain and Russia, and send agents into Canada, Mexico, and Central America to rouse a vigorous continental spirit of independence on this continent against European intervention. And, if satisfactory explanations are not received from Spain and France, Would convene Congress and declare war against them."
-a memorandum of Seward, April 1861.
"Now that the confederacy is about to be shorn of more than half its strength in territory, and more than a third of its population, it is necessary to repair the loss, else we would sink to a third or forth-rate power. By peaceable means or force, therefore, Canada must be annexed... such is the decree of manifest destiny, and such the programme of William H Seward premier of the President Elect"- New York Herald, February 1861
Was whatever Seward mentioned simply a bargaining chip to gain the favor of the Canadian people?
No. He expressed the view both publicly and privately on many occasions, and it's not even as if he was alone in this - the New York Herald enthusiastically supported the idea and was very widely read, and the Times of London noticed.
When there was the news of potential war, the Canadian population turned out enthusiastically for the militia muster.