I've been playing around with this very question, and I think an Entente victory is possible, but it requires the Entente to do much better in Europe. So if you don't mind a rather long post here's the scenario that I've imagined for how to get to an Entente victory.
The novels imply that the fighting in Europe goes much as OTL in 1914. It's been frequently mentioned in the World War 1 discussions in the After 1900 board that there was a moment during the fighting in France in 1914 where the BEF had an opportunity to advance into a gap between the German First and Second Armies and cut off the First Army.
I'm in the planning stages of my first Post-1900 timeline, and I've decided to focus it on the aftermath of an early Entente victory in WW1. I'm considering PODs like Plan XVII being rejected or the British Expeditionary Force advancing between the German 1st and 2nd Armies. Would either of...
www.alternatehistory.com
So lets say that same opportunity presents itself in TL-191 but ITTL the British commander proves more aggressive and seizes the moment. The German First Army is cut off and destroyed, and the rest of the German Army in the west is then forced to retreat all the way back to the Meuse to keep from getting cut off. The German invasion of France is a humiliating failure. So what follows from that...
In 1914 I think the biggest subsequent change is that with Germany having suffered a massive defeat in the west, Austria-Hungary not doing so hot against either Serbia or Russia, and the U.S. war effort not looking that impressive either (it's failed to knock Canada out of the war, has lost its de jure capital to the Confederates, and has the Confederates driving on its de facto capital), the war looks like it's going against the Quadruple Alliance, so the Ottoman Empire likely decides to stay neutral (and critically doesn't close the Straits.) This will make Russia much stronger since they can still get war supplies from Britain and France (and will have a lot of extra troops available since they aren't having to fight down in the Caucasus.)
The war in North America probably doesn't go that different from the novels for 1914 except I imagine the Socialists do somewhat better in the mid-term elections than they did in the books, since the war seems to be going worse than it was in the novels.
Come 1915 the lack of the Turks in the war means the British have a lot of options available for the troops that would otherwise have gone to Gallipoli. They can send them to Belgium, to Serbia, or to Maryland to help the Confederates. I'm going to assume they send these troops to North America (figuring the best way to help Canada is by helping the Confederates put as much pressure on the U.S. as possible.) Those extra troops won't be enough to achieve any decisive result in the fighting in North America that year, but they do mean the Confederates weather the Red Rebellion a lot better than they did on the novel. As such I doubt the U.S. is able to achieve the breakout they did from the Baltimore pocket in this timeline, so the Confederates aren't forced to withdraw from the Susquehanna. Custer likewise is getting less reinforcements than he did in the canon timeline (since more U.S. resources are needed in the eastern theater) and thus the U.S. advance in Kentucky proceeds even slower than it did in the novels.
Meanwhile in Europe, Italy seeing that the Entente seems to be winning the war decides to join in and attacks Austria-Hungary. The attacks aren't immediately successful, but they do put the Austro-Hungarians under increased pressure. The Germans try to help their allies by launching a counter-offensive in the east, but since Germany is weaker than IOTL (having lost First Army in 1914), and the Russians are stronger than IOTL (no Caucasus front, and they are getting war supplies from the British), the German offensive is nowhere near as success as their OTL 1915 campaign. (The Russians are still pushed back but no Great Retreat occurs, and German casualties are much higher than IOTL.) The French meanwhile attack in the west. Their attacks aren't particularly successful, but they do help keep the pressure on the Germans. Bulgaria, unwilling to risk entering the war with an uninvolved Turkey at its back, stays neutral, and thus Serbia holds out through 1915 as well.
1916 is the decisive year in Europe. Britain (having finally raised a mass conscription army and succumbing to French demands to emphasize defeating Germany over the U.S.) and France attack Germany in the west while Russia, Italy, and Serbia all launch offensives against the Austro-Hungarians. Romania then also enters the war and joins in on the attack on the Austro-Hungarians. The Russian offensive is even more effective than the OTL Brusilov Offensive (since at this point in the war Russia is doing much better than it was IOTL), while the Austro-Hungarians are weaker than IOTL and still have an active front (Serbia) that they weren't having to deal with IOTL. The Entente offensive thus proves too much for the Austro-Hungarians and their lines collapse. The Germans frantically try to send troops east to restore the situation and keep the Austro-Hungarians in the war, but that fatally weakens their lines against the British and French and thus the Entente also breaks through in the west. Around October 1916 Austro-Hungary sues for peace, and realizing the war is lost, the Germans do the same.
Meanwhile in North America, the war in 1916 sees the U.S. slowly grinding forward. Progress is slower than in the novels since there are several hundred thousand Commonwealth troops supporting the Confederate armies, and the CSA was able to crush the Red Rebellion much faster than in the canon timeline, so it is stronger as well, but the U.S. has crushed the Mormon Revolt and is advancing in Kentucky and Canada. And then the news from Europe comes of the collapse of first the Austro-Hungarians and then the Germans, just in time for the presidential election. With the war in Europe lost, and the U.S. now facing the prospect of having to face the entire power of the Entente alone, Eugene Debs' call for peace suddenly becomes a lot more attractive to the American voters. (And especially after a surprise Confederate offensive on the Roanoke front sends U.S. forces there reeling.) General Wood, sensing electoral defeat, begs TR to declare a national emergency and cancel the election, but TR refuses,. The election is held and Debs is victorious.
Although defeated at the polls, TR isn't ready to give up yet, hoping that with the 4 months he has left in office, he can still somehow win the war. Thus TR orders one last offensive on the Kentucky Front. Unfortunately, morale among the U.S. soldiers involved is terrible. (Having to fight in the middle of winter is bad enough but here the soldiers feel that the war is already over with the election of Debs and thus don't see why they are being ordered to carry out a pointless attack.) Thus after the initial assaults fail, U.S. Army soldiers start mutinying when ordered to attack again. General Custer's rather hamfisted efforts to suppress the initial mutinies, which involve mass arrests, drumhead courts-martial, and executions of the suspected leaders of the mutiny, just escalates the situation and as disorder spreads through the army , TR is forced to call off the offensive before the whole US Army disintegrates.
March 4, 1917 sees Eugene Debs sworn in as president. Debs' first action in his Inaugural Address is to call for a peace of no annexations and no reparations. Debs' second action is to issue pardons for all the soldiers that mutinied and to sack General Custer and any other officer that was involved in executing the mutineers. When General Wood protests these actions, Debs then asks for Wood's resignation. Debs will later admit that he half expected Wood to call in his soldiers and arrest Debs after that but Wood accepts his dismissal. (TR had actual met with Wood the night before and warned Wood not to move against the new administration with TR promising that if Wood did launch a coup, TR would be the first man on the barricades to oppose him.)
The Confederates reject Debs' call for peace. President Semmes was elected on a promise to make the U.S. pay for attacking the Confederates, so he does not think he can accept a status quo peace, and with the British no longer distracted in Europe, he feels that the Entente can win a decisive victory over the U.S. and force them to come to terms. Thus Semmes makes his own proposal which requires the U.S. to withdraw from all Entente territory that it has occupied, cede the District of Columbia and Maryland, recognize the Confederate conquest of Haiti, and pay a large sum of reparations to the CSA and Canada. Debs rejects those terms.
The British deploy their fleet to North American waters (the Imperial German Navy no longer being a threat) and begin sending hundreds of thousands of additional troops to Maryland. At the same time they warn the Confederates that the British people are ready for the war to be over, and thus a resolution to the conflict needs to be achieved very soon.
Anglo-Confederate forces attack with the Confederates trying to force the Susquehanna while the British try to crush the Baltimore pocket, but after some of the fiercest fighting of the war to date, the U.S. repulses the offensive. (US troops are no longer willing to take the offensive, but they are still willing to defend U.S. territory.) At the same time a massive naval battle between the U.S. and Royal Navy ends inconclusively.
The British then warn Semmes that they aren't going to lose anymore British lives trying to conquer Maryland for the CSA, and that if he doesn't join them in concluding a honorable peace with the U.S. the British will make a separate peace and leave the Confederates to fight on alone. Semmes' own military advisors warn him that Confederate (white) manpower is just about spent and that without the British the only way the CSA will be able to keep fighting the war is by enlisting black soldiers. Semmes' economic advisors likewise warn him that the Confederate States are pretty much bankrupt and can't afford to keep fighting absent massive subsidies from the British. Semmes thus accepts that the war needs to end and enters into peace negotiations.
The final armistice agreement sees the U.S. and the Entente nations withdraw from each other's territory though the Confederates still refuse to withdraw from occupied Haiti. (Debs refuses to recognize the Confederate conquest of Haiti but also does not let it prevent the signing of the armistice agreement.) Neither side will pay reparations to each other.
I think something like this is probably the most plausible Entente victory scenario though even here it is much more a victory for the European Entente powers than it is for the CSA. Indeed the Confederates are probably rather dissatisfied with their "victory" since they lost close to a million people, had Kentucky and Sequoyah devastated, and pretty much bankrupted themselves and all they got for it is Haiti (which is probably seething with rebellion.) The Confederates also probably feel rather betrayed by their British allies (even though it was British support that kept them from getting crushed.)