I cannot decide whether to acknowledge that you have made good points or to point out that you're being rather patronising and dismissive. So I'll do both. I'm trying to be helpful. If I miss a few things it's not out of any blind love for the Entente or foolish hatred of the CP. It's because this is an internet discussion board and I'm not expecting to be peremptorily told off if I dash off a quick answer.
Sorry, I didn't mean to be dismissive.

Yours analysis was otherwise rather good, it was just that it seemed to me it did not gave the problem of the limited French manpower pool exausting in having to face offensives on two fronts, due consideration. So you assume that the Italian intervention moves the Entente to call off major offensives, and this will make up for the elongated front fully in the long term. Sorry if I felt like anything like chastising.
I didn't expect to be chastised for that. IOTL the
Somme ate up over half a million Entente troops, and
Verdun almost another 400,000.
Ok, if they call off the Somme, they have half a million more reserves. However, Verdun was a German offensive, ITTL the German high command has better perspectives of victory, no reason they would renounce it, unless they need those men on the other front, say the added A-U manpower is causing the Russian Army to buckle under pressure, and they use the troops to make a deeper penetration into Russia. Nonetheless, the Italians are not going to be idle on the Alps, Cadorna, the Supreme Italian Commander, was a believer in the virtue of repeated offensives. So I guess that the vast majority of what they spare on the Somme, they lose on the Alps front, defending against Italian offensives. AND they still need to man that second front, too. That's why I see them suffering a manpower crisis by late 1916 at most.
Having sustained these losses IOTL the Entente was still capable of holding the OTL Western Front - though the fact that the Germans weren't attacking is obviously a factor in that.
Yes, although by 1917 the French Army came very close to collapsing with the Great Mutinies. of course, much of that came from the French Generals using offensive tactics that caused huge losses, and we are hypothesizing the Entente goes on the defensive on the Western Front, which might partiallyu defuse it. OTOH, they have another front here, which will increase them of at least 30-35%.
As for the impact on public opinion in America, invading a neutral doesn't look good, and there's little reason not to go for unrestricted U-boat warfare if one's already lost US public opinion. The major reason for not going for unrestricted U-boat warfare sooner was American opinion. That said, I'm not an expert on US politics of the Great War, and it could be that they'd just frown irritably while Switzerland's invaded.
Highly doubtful they will do anything more than frown. No American lives or strategic interests were involved. Freedom of the seas for neutrals was a national security interest.
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More A-H troops available for operations against Russia, possible earlier revolution.
Yes. IMO at least one year earlier. Which would mean the Germans have the opportunity to throw the great offensives of 1918 at least one year before.
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German armies engaged heavily in Switzerland and not at Verdun.
The crucial issue is, if they indeed do take the Switzerland option, where the front stabilizes initially ? Are the Anglo-French able to stop the CPs before they are pushed to the French frontier and beyond ? That frontier isn't fortified, it would offer them better breakthrough options than Verdun.
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No Salonika Expedition and no Gallipoli, freeing up Entente troops for redeployment to S France and Switzerland. Combined Total of these two IOTL is c. 25 divisions and change.
Well, it depends on when the Italians attack. OTL, it went on May 24, and Gallipoli went on April 25. Now, it is possible that negotiations between the CPs and Italy conclude quicker, anticipating their attacks. It is also possible that the Italian attack in South France occurs when the Entente have already heavily engaged in Gallipoli, and those 25 divisions are not available. Surely, by early 1915 the Anglo-French still have the manpower to committ troops in Turkey, North France, and South France, but this will them considerably further down the line. Now, it is entirely possible that in the evidence of Italian attack, they cut their losses and reimbark. This will increase CP pressure on the Russians although.
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Question mark over Somme & Verdun.*
Yes, and above all, the fact that if Verdun (or any other major German offensive) still occurs, it's quite simple for the Italians to launch an offensive of their own in South France at the same time. Italian and German Staffs had been collaborating since the 1880s, protocols for transferring Italian troops were in place before the war, I think that even if a Joint Command isn't established immediately, such coordination is plausible. This makes it possible that the weakened Entente defenses are penetrated.
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Italian troops heavily engaged in Switzerland and France, necessitating French defensive redeployment to S France.
Yes. At least 30-35% of Anglo-French forces must be redeployed there for the duration of the war.
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Question mark over American entry into the war.
Yes. The presence of Italy gives the CP considerable additional resources (both A-U and Italian) to focus on France and Russia. An hastened Russian collapse is possible. This makes it less likely the German High Command will greenlight unrestricted submarine warfare, and without it, USA will stay neutral till the end of the war.