1. US joins Allies of a roll of 1-2 for 1915
2. US automatically joins in 2016
Historically the Austro Hungarian Empire was very cautious with the use of their Navy in WW1.
"During the First World War, the navy saw some action, but prior to the Italian entry spent much of its time in its major naval base at Pola, except for small skirmishes. Following the Italian declaration of war the mere fact of its existence tied up the Italian Navy and the French Navy in the Mediterranean for the duration of the war."
Since the alt history has the French Navy out of the picture, and they were on the side of Italy in the game, I would have a roll of 1-3 that would activate the Austrian Navy and on 4-6 they stay in port for that turn.
The goal of the variant is simply to make a short game more competitive for both players.