Even if we assume the Italian and AH fleets work together, meaning that AH puts her fleet in line for destruction to help the very country the fleet was aimed against, Italy's near total dependency on sea travel due to lack of rail transport for just about everything in the area south of Rome, and Sicily and Sardinia, means that the only question is whether the economy for half the country is in ruins with or without the Italian fleet going down as well. So sitting on the defensive all but ensures disaster for Italy and a constitutional monarchy may not be able to hold up under those conditions.
I have to agree with Grimm Reaper on this point. The royal navy could strangle the italian economy easily, but it would take time to force Italy to submission through a blockade. And time is the key factor in this scenario.
Since the germans would have anyway invaded France passing through Belgium (triggering so the UK intervention), the question is if the french army could have stopped them, while having to man the italian border too. As Eurofed wrote, the battle of Marna was quite a close run OTL, so it would plausible that the central powers could prevail over the french army.
If the germans take Paris, what the french are going to do? If the army is still in a fighting shape, they could set up a goverment in Borderaux and try to retake Paris. But if the army is destroyed (like happened in 1870 and 1940) they could even sue for peace. And with France out of the equation, the entente would be in some dire straits.
So in this scenario, the war will be necessarily quick. The first side to lose one member due economic attrition or conquest will probably lose the war too.
As for the USA I think that they would follow OTL policy and don't get involved initially. They won't feel any need to meddle with european affairs (not anymore than OTL), but not for the various ethnic groups efforts, but simply for lack of true interest. OTL it took three years of naval warfare and a lot of investments in the entente nations to trigger the american intervention.
The ottoman empire will probably stay neutral too. Enver pasha could force the war only through the Goeben arrival, but ITTL the ship would probably seek shelter in an italian port. The sublime porte could be indeed tempted to join the entente and get back the territories held by the italians, but I think they would prefer neutrality.