Apart from being a totally unrealistic scenario, here are some points:
By this late stage in the war it is unlikely the US could win.
It wasn't prepared for war and had a small military, so it would take time before it could build up its strength to threaten the Allies. It might even suffer damaging defeats to the battle hardened Allies.
Its navy was no match for the RN so it would not be able to break the blockade of Germany or provide much material support, and would likely suffer a blockade of its own.
Its financial, economic and business interests were on the Allied side, so the US damages its own economy by going to war against the Allies.
Even if it somehow won WWI as a Central Power, it loses in the long term - it was in the US economic and geopolitical interests to support the Allies. This is why it supported the Allies in two World Wars and Western Europe in the Cold War. Like Britain's long standing foreign policy, a Europe united under Germany (or any one power) will shut the US out of its markets and become a rival superpower in itself. So the US doesn't rise to prominence in the 20th century as it does IOTL because it stupidly went to war contrary to its own interests.
Naval geography lesson.
O-class submarine
Displacement:-521 long tons (529 t) surfaced; 629 long tons (639 t) submerged
Length: 172 ft 3 in (52.5 m)
Beam: 18 ft 1 in (5.5 m)
Draft: 14 ft 5 in (4.4 m)
Installed power: 880 bhp (660 kW) (diesel); 740 hp (550 kW) (electric)
Propulsion: 2 × diesel engines. 2 × electric motors
Speed: 14 knots (26 km/h; 16 mph) surfaced, 10.5 knots (19.4 km/h; 12.1 mph) submerged
Range: 5,500 nmi (10,200 km; 6,300 mi) at 11.5 knots (21.3 km/h; 13.2 mph) on the surface
Test depth: 200 feet (61.0 m)
Complement: 2 officers, 27 men
Armament:4 × 18 inch (450 mm) torpedo tubes
1 × 3"/50 caliber deck gun
She doesn't look like much, but she could refuel at sea, and she was underwater FAST.
What's worse is that while the O-1 took 16 months from keel to wet, this was a design that could be cranked out on assembly lines like 4 stackers in about 6 months.
The S-class were true Atlantic boats but you go with what you have in 1916.
Still saying
this is strictly ASB for political and economic reasons, but militarily as a what-if, the Entente would be cutting their throats if they made an enemy of the 1914-1918 United States.
1. Wilson; a butter knife of a president, was not too stable emotionally.
2. Geography and fleet sortie radii favor the USN massively.
3. Standards are no joke. The RN has 8 BBs that can withstand them. America's weakness is cruisers and escorts.
4. The SLOCs which are difficult to impossible for Germany to attack are easy for the USN to shut down. There's not a thing the RN can do about it.
5. Britain's option is to use Japan as a distraction. But why should Tokyo not cut a deal over China? Alliance of convenience. Means trouble for the US later, but that's coming anyway.
Add this:
1. 1/4 of the Entente's finished ammunition, weaponry and supplies were US made.
2. The Entente was dependent on S. America for oil, beef and nitrates. The US allowed that trade. Yes, I wrote allowed.
3. The US will suffer default on European bank loans anyway (Part of the 1929 Crash) and Wilson is just the kind of maniac who would say "Fudge it, let's go to war." when the Entente threatens default.
About Canada...
It would be nasty. Canada could recall her soldiers serving in France. Assuming they aren't killed crossing the Atlantic, that means the Entente loses ~500,000 excellent troops. The Entente cannot afford that kind of hit. If those Canadians stay in France, then where is this "mythical force" going to appear that will give the US northern border so much trouble? Again it would be NASTY. Halifax becomes the principle American target with Winnipeg next. Grab the nose and stab the belly, the cow dies. Takes a year using WW I methods. Casualties enormous, at least a half million dead for both sides. Insane.
Result? Canada is gone.
How is that submarine campaign going? Depends. How soon will Britain be BANKRUPT? The US doesn't have that financial problem. She's a self contained economy.
These things (see map) were well known in London and Paris. Those guys were not insane (Well crazy for allowing this war and ruining Europe.) but at least they knew enough geography and had advisors who could show them
what happens if they go completely bat-guano whacko.