So getting Italy to agree to separate peace is possible, but only after Caparetto and only if the Habsburgs offer the Treaty of London territorial concessions.
I was thinking more about the sixtus affair and no the full Treaty of London was out of the question and even Rome knows it, it was more a negotiation tattic...the problem was that Wien was not ready to give up anything for peace.
If A-H was willing to offer the Treaty of London concessions before Italy’s entry then Italy would likely have joined the Central Powers. Even after they had joined, receiving that territory would have been enough to get them to leave or even swap sides - no Caporetto necessary. But that was never on the table. A-H wouldn’t stand for it. If the Germans tried to strong arm A-H into handing over all of Treaty of London territories then the A-H would try to seek a separate peace. Trentino and Trieste is the maximum that A-H would voluntarily part with, and even that would be difficult for Germany to force A-H to agree to.
If Wien at least have conceded what promised for neutrality (maybe just a little more), both Paris and London will had the political capital to strongarm Italy to accept it, maybe with the stick to stop supply and financial aid and the carrot of some further compensation along the road; but without A-h being ready to give up even an hinch of his territory there is no way that they will have the possibility to force Italy to go for it and try will have probably broke the Entente.