I think that by early 1915 Italy had seen that France and Britain hadn't been knocked out of the war in the initial offensive and were now bringing more and more material and financial resources to bear and while Russia struggled against Germany she had struck grevious blows against AH. As a result Italy was more inclined to think the Entente could deliver their promises while the CP could promise the world but not deliver.
So I wonder if a change in the fortunes of war significant enough to make Italy think the CP would deliver on their promises be significant enough for the CP to win the war without Italy anyway? Something like winning the Race to the Sea, or the HSF getting a big early win against the RN or a bigger success against Russia.
The fortunes of war - well, the perceptions of the fortunes of war - certainly factor in to a decision like Italy's. Obviously, all but the most belligerent (at least, of a secondary power like Italy) would blanche at jumping onside with an alliance which is blatantly on the ropes. If any of the developments you muse on take place, you're right to suspect that Italy's government is going to sit it out.
And I don't think that the Entente looked like war winners in winter/spring 1915. They just had kept fighting on to the point where they no longer necessarily looked like war losers. The war was a stalemate at that point, basically...
Italy's decision to enter the Great War is a strange one, and not just because it plainly worked out to be a bad decision for her; it was also the least
pre-determined decision for war of any major combatant power. Mostly, it really seems to have been the work of Baron Sonnino, operating in a momentary power vacuum of sorts. It's so easy to generate a P.O.D. where Italy ends up staying neutral. And even for Sonnino, it was possibly mainly because there was a momentary shift in political heft to the liberal nationalists (who preferred to work up their irredentist jones against the Austrians rather than the French) and a renewed political instability which Sonnino hoped to solve by winning a territorially advantageous war.
The Italians never offered to go to war against the Entente The most they offered the Austrians was neutrality. They began negotiating with the Entente to enter the war in September 1914.
Which gets us back to the O.P., doesn't it?
By 1914, an Italy honoring the Triple Alliance and going to war was definitely less likely than joining the Entente, because over the previous decade, developments had occurred that made the Alliance make less sense for the Italians. 1) Italian seizure of Libya and the Dodecanese from the Turks put them at crossgrains with efforts by Berlin and Vienna to court the Ottomans; 2) Italy felt aggrieved by not being consulted over the Austrian annexation of Bosnia, and the extension of its power deeper into Balkan territory she herself coveted; 3) Italy had worked out its trade disputes with France largely to its satisfaction. And, finally, Britain since 1905 had drifted increasingly into the Entente orbit, and British belligerency was a potent naval threat to Italy: a threat which made Italian entry into the war against the Entente something to think hard about; not a decision to be made lightly.
But then there are the fundamentals of Italian irrendentism, too, and these had always made Austria-Hungary a more attractive target. The ambitions of Italian nationalists against France were limited chiefly to Tunisia, Nice, Savoy and Corsica, and really only the first two, since these were the only territories to contain any significant number of Italian speakers. Whereas Austrian territories of interest - Trentino, South Tyrol, Istria, Dalmatia - contained nearly a million Italian speakers, and had a deeper cultural connection for Italians. The bulk of these territories had been under Venetian rule for centuries.
Berlin and Vienna were aware of most of this, and this is why they were content to lobby Rome for just neutrality, which on the whole would have suited the Central Powers' purposes best anyway. Had things worked out a bit differently - had Sonnino choked to death on a chicken bone in the spring of '14, say - I think that they likely would got their wish, and maybe without any formal deal.