If Germany causes France to throw in the sponge sometime between April, 1917 and late spring 1918 when significant numbers of US combat forces arrive (remember support forces came first, and combat forces followed and had further training in France), it's game over. I expect the Germans will let the US & British to remove their forces and equipment, and the French will be tasked with speeding this along. The US and UK cannot continue in western Europe if France (and also Belgium) surrender. As noted it would take the USA 12-24 months to produce in quantity items French industry was producing for the US military (artillery, aircraft, etc). Until then the US Army and air service are in no shape to go up against CP forces. Britain cannot produce adequate surplus to equip the US military. Russia is out by now, and with France out the CP can put as much as they want in to the Balkans & against Italy - the latter won't last long in this scenario. Sure the US can send forces planned for France to Italy, this will take 3-6 months to shift, they will be light on heavy artillery and aircraft, and will be facing a geography where they are attacking experienced well entrenched troops on the heights. The mountains represent a formidable barrier that can be held readily by CP exacting a hugely disproportional cost on the Entente. I doubt the USA will be happy to see those sorts of casualty lists for this cause.
Because the CP has a surplus of troops now with Russia and France/Belgium the Ottomans can be reinforced, and frankly the preservation of the Ottoman Empire is not pivotal to Germany or A-H. IMHO Italy will take a quick peace if they can avoid any significant territorial concessions, and since the CP have no significant demands some border adjustments and/or a DMZ would be something they could offer.
As far as the blockade, this is 100% dependent on the British. If they keep fighting there can be a blockade, if not the USA can't do it by themselves from bases in the USA. The Navy isn't large enough, and I can't the US stopping British ships from trading with the continent if the RN isn't complicit. Also, now Germany (and A-H) can get resources from France and Russia. Would the UK and USA let France starve by maintaining the blockade as food is shipped to Germany - I doubt it.
Especially if the UK and Germany conclude a status ante bellum peace or something close to it, the USA simply can't fight Germany and vice versa. If Germany takes some French possessions in the western hemisphere there is the potential for Monroe Doctrine issues, but since the issue is transfer of a European owned territory from one European power to another, this is probably not anywhere close to enough to continue the fighting, whether or not htis would be an issue in the future is another matter.
Bottom line is in 1918 a war between the USA and Germany (+/- A-H) and nobody else is simply not happening.