Absolute best case for Central Powers in Italy is taking Poe River valley in 1916. This is border line ASB but possible with enough Entente errors. Realistic best case scenario would be taking maybe 1/3 of Poe River valley before British reinforcements block the advance at some terrain feature.
So handwavium the unlikeliness of the decision to attack and series of blunders, here is roughly what ATL looks like.
- Over winter, Germans decide Italy is weak link. Probably also look at warmer weather in Italy and flooding in Flanders.
- Feb 1, instead of Verdun a grand offensive into Italy is launched. Mostly German troops.
- By April 1, offensive is bogged down due to British and maybe French rushing troops to area. Interior lines of supply and communications work for Entente too.
- Little bit after this French and Russian will launch big attacks. Now for CP to be ok, the A-H needs to have left enough troops in east to bog down Russian offensive to a blah attack for little gain. Flanders is flooded and British were taking longest to get ready, so Somme type attack is months after OTL. French launch attacks, Germans will have to hold. Germans have to also hold as OTL second phase of Brusilov is launched against Germans. OTL successful attacks were diversionary attacks in the plan.
Now this all looks pretty good. Might look better than OTL to you. However, it is not really except I made the Busilov offensive in the east fail which is really unrelated to this POD. In reality, the Brusilov works, and A-H will never again launch an offensive attack without German help. And if USW is resumed and USA still enters war, Germany still lose the war in Flanders. What really changes here is that the French don't have Verdun which had such large political impacts in post-war French politics. And Italy being crushed by Germany and losing a few field armies to encirclement or attrition (few more field armies than OTL) has huge impacts on Italian politics.