A Neutral Italy during WW1 would clearly favour the Central Powers. Austria would have a front less to man, and after Russia collapse, assuming that this is not butterflied away, the central empires would have more men to send against the entente in France. There would be also more manpower available for industries and agriculture, too. The actual end of the war would depend on the resistance of the entente and the eventual US entry into the war.
Having Italy stay neutral is actually not so difficult. The large majority of the nation was for peace and only a minority desired war against A-H. The italian entry in war was decided though by the goverment ignoring the parliament, with the blessing of the royal court. Basically, what you need to have Italy neutral is changing the mind of Vittorio Emanuele. Without his consent, the secret treaty of London would have never been signed.
Italy was Central Power before WW1, then they betrayed the Central Powers and later (in 1915) entered the Entente.
Yourworstnightmare, do you know the difference between a defensive alliance and an offensive one? I guess not, since you keep repeating things like that. Italy wasn't bound to enter war because Austria started it, period.
The italian partecipation to the triple alliance was anyway doomed, since the several problems that Italy got with A-H, mainly the control of the Adriatic sea and the expansion in the Balkans.
However Italy would probably side with the winners near the end of the war.
Another of your pet ideas...And exactly when they should join the war? If Italy join the CP after Russia collapse, the entente is going to crumble. But the real question is: would Italy join her former allies without receiving the austrian land that italians claimed? And would Austrian ITTL agree on this?
Greece was neutral pro-Central Powers so the Allies blockaded the capital and forced a change of government that became pro-Entente.
The idea of Entente exerciting pressure on Italy is dubious, since they could not afford the risk of Italy of joining the central powers... The french were already stretched thin without having a second front.
Negative effects in 1917: Italy had a negative role for the Entente during 1917. During spring and summer, Italian X and XI battles of Isonzo had no effects on the general course of war; after Caporetto, French and British troops had to rush in Italy to avoid its collapse.
This is not completely true. The entente sent eleven division, IRRC. Not really a huge expedition corp and not even a relevant one. The Austrian offensive was actually halted by their logistic, that was unable to provide them with supplies. The german and austrian troops could not launch coordinate attacks on the italians and had to stop the advance. This gave enough time to the italians to reform a line along the Piave.