Q How realistic were von Papen's plans to invade Canada using “German cowboys” during the Great War?

Interesting. But. Not. "Not very realistic" was what analysts judged it later, his superior at the time, and the man himself in hindsight.

There were merely a few organizational details needing clarification. Wirth suggested that the entire operation should be placed under the command of the military attache in Washington, von Papen, and that the secret funds of the Prussian war ministry should provide 100 million marks to cover expenses which, given the importance of the operation, was little enough. The author of the memorandum, aware that the utmost secrecy was essential to the success of the operation, made the ingenious suggestion that these irregular troops should be dressed as cowboys so as to avoid notice. Were the Canadian government to protest the approach of 650,000 heavily armed cowboys with foreign accents marching towards their border, it would simply infuriate the American authorities and help to drive the United States into the welcoming arms of the Central Powers. Thus Canada would be destroyed, the entente weakened, and Germany would gain a new and valuable ally in the United States.

That sounds quite like a recipe for success?

The German foreign office was sufficiently interested in this nonsense to pursue the matter further. Of particular interest to them was the question of whether the traditional garb of the cowboy could be construed as a uniform. Their legal department researched the problem and decided that it could not possibly be considered a uniform under the definitions of international law. It was then suggested that badges of rank and regimental insignia be affixed to these outfits on crossing the Canadian border, thus in a flash converting work-clothes into military uniforms. Much legal expertise was devoted to this tricky point, but in the end cooler heads prevailed. What the Germans never seriously questioned was the possibility of finding 650,000 volunteers who were prepared to attack Canada, which showed their complete misunderstanding of the realities of North American life.

Martin Kitchen: "The German Invasion of Canada in the First World War", The International History Review, Vol. 7, No. 2 (May, 1985), pp. 245-260, (For the complete sad story of ambition: jstor

There was this trusty, and convincing, Herr von Papen:

This development, in which Canada found itself usefully positioned between Great Britain and the United States, was abetted by the help received from both countries in combatting the threat of German espionage. This threat arose from across the border, owing to the flamboyant if rather farcical plotting of the German military attaché to the United States, Capt. Franz von Papen. As Martin Kitchen, the chronicler of the espionage thriller, has shown, von Papen took his instructions to disrupt essential communications in Canada seriously and bent his energies to finding suitable minions to carry out his task. In the end, von Papen was unable either to find saboteurs or to conjure up a disgruntled army of German-Americans. The whole plot fizzled out following the capture of a lone German agent, Werner van Horn, after a bungled attempt on a Canadian railway bridge near the Maine border. The best epithet on the whole affair was von Papen's own: "I must admit that I … did not act very intelligently."

Although the damage inflicted by German sabotage was insignificant, the threat itself served to boost the status of intelligence work. The Canadian government pressed the Royal Canadian Mounted Police (RCMP), military intelligence, and even private detective services into action against potential railway bombers. To bolster its own meager counterintelligence resources, Ottawa relied on information from British and American intelligence services, both of which were keeping a close watch on the activities of the German embassy in Washington.

Wesley K Wark: "The Evolution of Military Intelligence in Canada", Armed Forces and Society, Vol 16, No 1, 1989:

who is on the right (only to be trumped in "rightousness" by Hugenberg:

Tangential: German plans for invading the United States were drawn up as emperor Wilhelm wished since the 1890s. In this case for Canada, the first war-time idea as in question here was submitted to the German foreign office on 29 August 1914 by Emrich Herzog from Vienna. The Germans called Vienna immediately to inquire about this unknown man and the answer was that Herzog was known as a delusional patriot, "constantly pestering officials with unrealistic ideas" and that he was best ignored. What Vienna thought crazy, Berlin called it creative.

On November 10, a Mr Wirth from Munich wrote likewise, claiming to be an expert on North America as he had spent time in Canada. He fantasised that a US based invasion of Canada would be easy, as that would meet the approval of Americans, especially Irish-related, and that already 100000 German army reservists would stand at the ready in North America with 250000 easily to mobilise. Adding to those two numbers the assumed 300000 Irish rebels we arrive at the fantastical 650000 jubilant troops. All what would have been needed then was ordering the chief of operations in Washington von Papen to do that and give him the funds to start action.

Martin Kitchen: "Militärische Unternehmungen gegen Kanada im Ersten Weltkrieg", Militärgeschichtliche Zeitschrift, Volume 7, Issue 1, 1970, (DOI)

Apparently, ambassador Bernstorff just canceled all efforts into that direction.