There is still Halifax and in the Pacific, the British should still have Prince Rupert, a deep seaport (actually I read the third deepest natural harbor in the world) connected to the Grand Trunk Pacific Railway that had just opened in 1914. Geography alone makes it very unlikely an American invasion would have captured it so far.
Also, how is that US forces could operate a build up of the scale required for a blitz invasion of Canada without being noticed. Even with the border locked, leaks are possible and at least the diplomatic crisis would have forced Canada to put its forces on high alert, freezing the deployments to France and putting to use the divisions it was in the process of training and equipping for a deployment in the fall.
Here's a pdf about Canada mobilization during the great war with some specifics:
www.canada.ca
Critical analysis of Canada’s recruitment for the war effort can be grouped around three main themes. The first is the government’s attempt to raise an expeditionary force that proved too large to be maintained by voluntary enlistment. As a result, conscription for overseas service had to be...
encyclopedia.1914-1918-online.net
The document from first link does mention that Canada had at the beginning of the war in summer 1914 about 3,000 regular army soldiers and 74,000 men in the militia organized into six divisions, which were all put on war footing afterwards.
Figure 1 from the second link show enlistments during the war in Canada, and by a rough visual approximation, I'd put the average rate at 10,000 volunteers enrolling per month over the August 1914-June 1915 period, so that's a further 110,000 volunteers trained at various degrees on top.
The only force Canada had sent so far to England, that was in late 1914, was about 31,000 strong.
So, by the time Roosevelt declares war on Canada on June 28th of 1915, Canada should have around 150,000 militia and volunteers under arms by OTL standard.
Here, I wouldn't see the US forces having and avantage more than 5:2 without draft, and with the boost of enlistments after the Yellow Rose incident only two months long.
Of note, it is mentionned in the first source that Canadian forces had already a good experience of warfare from the contingent that was sent to fight in South Africa against the Boers. I think that experience outweighs that on American side of the war with Spain. That also provides the Canadians with a blueprint for asymmetrical warfare if need be; and if that goes so far, and the Americans react as they did in the Philippines, they will have a very hard time occupying Canada.