And then the Americans weigh in
British intervention could have brought the war to a swift end, and not to the liking of France or Britain. The Americans are not known for taking interferance with their ships lightly, and their trade with both sides was very profitable--and kept the civilians on both sides from starving. (of course, the ongoing supplies of military equipment may have made the military carnage far worse, but I doubt it--if people wnat to kill each other, they'll find a way.)
Given the Royal Navy's track record with neutral ships, they would try to prevent shipments of war materials to Germany or the neutral powers bordering it. Britain would probably only back down under the threat of dire consequences. The first American military escort of American merchants might do the trick--or might lead to an accidental exhange of gunfire at sea. At that point, the best outcome for Britain is a complete acknowlegement of American rights, a large indemnity for already seized ships, and quite possibly a partial embargo, with the US inclined to be neutral in Germany's favor.
Worst case for Britain--they think the US is not willing to go to war until the Declaration of War is delivered. Everything in the western hemishpere is over run by American troops, American industry revs into high gear.
The Royal Navy will be able to dominate either the American Navy or the HighSeas Fleet, but not both.
The bad part for Germnay is the end of American supplies--the Royal Navy's light forces will be able to maintain an effective blockade, given the recently revealed documents on Royal Navy planning.
It is also interesting to see if Britain would have released the Ottoman Empire's newest ships had it entered the war. As it is, the month long hold on them after completion brought serious tension between the two nations, and explains why the Empire's next battleships were of American build, and American companies built the naval facilities and factories that allow the Ottoman Comonwealth to dominate the eastern Med.
Most likely, the Danish Copenhagen class would also have been built inthe USA as well, since Britain at war wouldn't have built foreigh ships at all.
Had Britian survived the war, and a major battle broken out, it is likely that the instability of their cordite would have been discovered then, rather than in the World War.
As far as a World War, if America did enter the war, I don't think a world war later on could happen, the industry of Germany and America (with Canada) would preclude any challenges by anyone. I think that America and Germany could manage to stay on good terms for some time, and the rest of the world would have to accept Pax Americana-Germania.