UK could crush Italy? I mean Austria and Germany did endure the blocade for 4 years. And a close blocade of Italy is very unlikely - the british still need their fleet in the North Sea to maintain an owerhelming supreiority against the germans - also the Ottomans remaining neutral most likely means that they did receive the ordered dreadnoughts from the british which would make the Admirality even less inclined to send further ships to the mediterranean. Im however unsure how the Austrian and Italian navy would compare with the French and whatever the british are willing to spare for the theater.
As for italian war goals: Nizza, Savoy and Tunisia, maybe Corsica. And maybe Austria agreed to hand over some of the Italian demanded territories in exchange for Italy keeping its commitments while promising all the above.
For effects:
1.Austria would be incredibly better off. No Italian front and without Italy and a new western front in the south the Western powers couldnt spare troops for Balkan adventures - also supplying them would be much more at risk with the Mediterranean contested so likely no Balkan front. This means that instead of 3 fronts Austria is fighting only against Russia.
2. Bulgaria has no enemy in sight - knowing the bulgarians they would be reluctant to transfer troops to fight Russia. They however would pose a threat to Romania who in this scnenario would be much more reluctant to join the entente because of the bulgarian threat. If they still join the entente I expect that they will loose swiftly.
3. Germany: That the british and french would need to send troops agains Italy to a southern front would weaken the line in the north considerably. However without the ottomans in the british could use the troops of OTL Ottoman fronts in Europe. Russia should too have additional troops available howeve I dont think that they would leave the Ottoman border undefended. If the need arises to prop up the Italians Germany could send the same troops it used OTL to prop up the otto's and austria in Italy so that should be enough to prevent an italian defeat.
4. Russia can trade with the other entente power though the mediterranean was already a very easy territory for U-boats OTL and with Italy on CP side likely much more so. So I expect very heavy shipping losses.
To summarize: Im not sure the war would end sooner and not at all sure the CP's wouldnt win.
Italy requires much more resources to be imported than German and Austria so they would be in trouble quicker than the other central powers.
The combined Austrian and Italian fleets would be more than matched by the French and British fleets who OTL concentrated on bottling up the Austrians and keeping a close blockade of the Ottomans. Here they can play against the Italians and still not have to weaken the Grand Fleet in the North Sea.
Uboats where not the weapon they where a generation later. They where only useful when operating unrestricted against unconvoyed merchant ships. Most vital shipping was convoyed and attacking everything else risks US entry into the war as OTL. Further Austria had Uboats but siblings where never that significant OTL would Italy’s Uboats really be that much more decisive? Arguably it could accelerate anti submarine warfare including earlier depth charges.