1) Britain, France and Russia were economically stronger than the Central Powers. OK so they cannot tap the USA, Canada or Italy for support but they ought to win in the very long run. Given it was 1918 on OTL you are looking well into 1919 may be even 1920 for an armistic in Europe. All the CSA and Canada would have to do is hold out until then.So what would cause the CSA and the Entente to win the war? I can see Custer's offensive to take Nashville falling, leading to the US losing the war, but how does the Entente win in Europe?
2) The USA not build a workable barrel, but instead go down the German A7V route. However, given US automotive prowess at the time I would expect them to still build the barrels they did. That means figuring out how to keep them out of Custer's and Morrell's hands.
3) The Red Uprising nipped in the bud. Given the US will supply the Reds weapons, it is going to happen although it could have a lower impact.