If I understand correctly, this note is NOT being sent currently, but is a contingency prepared, so that IF the USA gets into the war, the Department of State has its instructions for dealing with the British government marked out clearly. USA in war without such concessions means that the USA is not sending its troops into the meat grinder, but working on the peripheries and at sea.
If this note is sent, it's a starting point for negotiations, so that the USA doesn't end up with a mass of dead citizens and no gain. The beauty of it is that, should the USA end up in the war on either side, it stands to gain something. Without some sort of promise, there's nothing the USA can gain by fighting the Central Powers...
Of course, extorting (as the British would see it) territorial concessions in exchange for helping the Brits fight Germany could poison the relationship forever, far more than the USA joining the Central Powers might.
The mere thought of the USA as an enemy should frighten the British; Canada is very vulnerable.
Great timeline, and in this Great War, a lot of lessons have already been learned...
HMMMM...having seen more modern warfare, has anyone already been thinking of ways to break through trenchlines, via tracked armored vehicles, Storm Troopers (Great War version, not SS!) or gas warfare?