Stay on topic, the thread is that of naval clashes between the USN and the RN.
The chances of the US and the UK slugging it out old style over a mere trade issue is non-existing. Even more so if the UK are already very busy echxanging rudnesses with the Germans all across Flanders while being simultaneously engaged in a staring contest with the Kaisers fancy toys, better known as the High Seas Fleet.
i seriously doubt that the Royal Navy command would simply sit on their hands
If the RN decided to actually do something, the US east coast will be one big target practise area. Having a look at where the majority of US shipyards are, that would have a slightly disrupting effect on the US fleet building program, which in turn would also slightly delay your envisaged completion date of 1920/21. By, say, 2-3 years?
You are stating for the record that Britain wont interdict any US convoy to Germany. Britain wont defend a blockade it herself enacted. US covoys make it to Germany unmolested.
You are stating that Britain's top brass is completely happy to allow arms shipments to be allowed to offload in Germany and that this eventually lead to the untold slaughter of millions of its own troops and allied troops as well.
You are conveniently ignoring that I also assume that such an escort deter the UK from searching the ships. It would also deter them from picking a fight with the US Navy. In the end, without the US joining the Entente, the latter will ask for negotiations sooner than later, facing the Central Powers in France and Italy and having the US defending free and uninterrupted trade at point blank range, they won't have much of a choice.
In case of the US so much as
threatening to cough in the direction of London, this would cause a whole series of sneezes, running noses and headaches within the UK top brass, especially once Russia leaves the international competetition on who gets more men killed by a foreign power. In that situation, the whole potential of facing the Huns in France, the Austrians in Italy and the US in... whereever they chose to have a party, is so desasterous that the UK and especially France will want to get out of the whole affair right there and then.
Bottom line being: The potential material damage to the Entente war effort was so huge, the Entente wanted to avoid such a threat to materialize at virtually any cost. The US forces could easily occupy the Entente colonies in the Carribean Sea and the nothern parts of South America. With a little preparation, Pacific islands and Far East posessions would be within reach. Canada would be prone to recall her units from abroad and that would likely not be enough to stop the neighbours with the weird accents, thus removing Canada from the list of belligerents and depriving the French battlefield of four (Canadian) divisions. And all these are just the short term effects.