Leaving aside the likelihood of the US joining a European alliance structure in the first place,
I agree, that would require something like France intervening in the ACW or having a limited war with the Union over Mexico. However, it is still possible to bring the USA in WWI to the CP side even without an official alliance.
wouldn't the most likely outcome be that both Italy and the US sit out for a while, citing the defensive nature of the pact?
Indeed. But it is still quite possible to bring Italy to the CP side if A-H agrees to cede Trento and Trieste (negotiations were underway in early 1915, make them successful if Germany persuades A-H to comply). If that can be done, Italy can be brought in the CP quite easily, they has plenaty of irredentist and colonial cliams against France.
As it concerns the USA, you need a pro-C.P. President (say, Teddy Roosevelt wins the Republican nomination in 1912) who takes offense at the UK naval blockade (much like Madison did in 1812) and some naval incident to escalate tensions between the UK and the USA.
Other diplomatic changes are that the Entente will probably promise Mexico the moon in exchange for a second front in the Americas. Canada is presumably still stomped flat in a hurry, and Mexico is dealt with second.
An Entente Zimmerman telegram ? Oh, that will still bring the USA in the war like a jaggernaut.
An interesting note is that the US developed modern submarines first (the Holland) and would likely have a much better submarine arm than Germany if they've been contemplating the possibility of a general war. British naval strategy is going to be a mess, especially if the US bases sub squadrons in the Mediterranean and takes over that theater, allowing the Germans to pack all of their U-boats around the British Isles. Their #1 mission, however, would be to prevent the US and German fleets from linking up in the Atlantic at all costs.
Well, assuming Italy enters the war against France in May-June 1915, and the US anytime in 1915 (let's assume several months for naval tensions between US and UK to escalate), I would deem that for at least a year to an year and half the US land forces are engrossed to conquer and pacify Canada, and Mexico, too. Historically, Mexico was a hotbed of revolutionary unrest in the 1910s, and the trans-border incursions of Pancho Villa did bring the US to an undeclared war with Mexico. If an interventist Administration gets the suspect that an hostile UK might land forces there (as it would be wholly capable to do, differently from Germany OTL), it might well decide to escalate it to a declared war and swamp Mexico with troops, too, to nullify the threat. 1915 USA surely has the military potential to wage a two-front land war on both borders. However, readily-available troops will be entirely engrossed to conquer Canada, and later Mexico. I do not know if in such a war, a Roosevelt Administration would still activate the draft or use a volunteer force as in ACW (at present, I do not know what the OTL stance of Teddy on the draft was). Probably, war which starts with "liberation" of Canada will be rather more popular than one that immediately requires an intervention in Europe (even if such will be obviously necessary later, too). Surely, a lot of volunteers can be recuited among German and Italian Americans.
Anyway, recruiting and training a sufficient expeditionary force to hit the Entente overseas, in addition to the occupation force for Canada (probably the US will meet as much resistance to their conquest from loyalist Canadians as they got in Philippines, even if they are not ever going to relinquish Canada after they got it), will require at least a year.
However, this still quickly deprives the UK of the resources and manpower of Canada.
As it concerns the US Navy, it will be immediately free to act against the British. A part will be necessary to protect the coasts against UK harassment, a part will be necessary to cut communications between Britain and Canada. The rest will be free to support the Germans in the North Sea and the AH/Italians/Ottomans in the Mediterranean. Probably the Entente will be forced to give up the Mediterranean quickly and largely concentrate around the Home Isles to protect their supplies from the combined US-German Fleets. Surely, the ability of the Entente to keep a tight naval blcoakde of the CP commerce will be greately diminished in this scenario.
I do not know if, once Canada will be won, whether the combined US-German fleets would be enough to counterblockade Britain severly enough to starve it into surrender. However, anyway, in about a year, the US Expeditionary Force will be ready to hit the Entente. Where the President is most likely to send them ? They could hit in many different theaters: France, Northern Front, Alps Front, Russia, Poland/Galicia Front, Caucasus Front, UK, India, Egypt, Malaya, open a new Russian Front at Vladivostok (unlikey if Japan is in the Entente)...
The UK has lost Canadaian manpower, and all the troops stationed there, and her ability to relieve France has been significantly, but not radically, diminished. France is facing Germany and Italy with major British support, Russia is facing Germany, A-H, and the OE. Serbia was wiped out in a few months (A-H + Italy + Bulgaria). Bulgaria helped to kill Serbia. Romania is a wild card, might be a cowed neutral, have jumped on the CP side, or entered on the Entente side and be wiped out in a few months. The Entente is probably still standing but somewhat teetering by this point.
