regarding the defensive attitude, that was not the french mentality at the beginning of the war: see plan XVII (and there are not trenches on the pidemont border).
There were a lot of mountains, even better. I'm not saying that the overconfident French would not do try an offensive on the Alps. I'm just saying that a) they would still try to do a double offensive, in Alsace-Lorraine and on the Alps anyway, manpower problems be damned. They were THAT obsessed about recovering bloody A-L b) this wasn't Napoleon's time remotely anymore, military technology was radically different. Modern artillery and machine-guns made the defensive in strong positions much, much more effective. The French would get a double bloody nose in A-L and the Alps (if you doubt it, Alsace and the Rhineland were a place where the French armies broke through without excessive effort, not just the Alps, in Napoleon's time, and got a bloodbath in 1870 and 1914; the stategic picture was made radically different by the Industrial Revolution).
Even ruling out a compliant Switzerland
Even admitting that an ASB butterfly would remove the proud attachment of the Swiss to their neutrality, Switzerland would have CP powers on three sides. In the very plausible case that the Entente loses the war, for Switzerland giving free passage to the French army would be like sending a "Partition me" letter to CPs. Of course, it is quite possible for France to
invade unwilling Switzerland. It would be basically OTL Belgium in reverse. The outraged Swiss would fight the invaders tooth and nail in a terrain quite favorable to defense, giving the CPs ample time to send their troops to shore up the Swiss defense. In the end, it would result into France grabbing a useless slice of western Switzerland, seriously tainting the Entente's cause in the face of other neutrals, and making its manpower problems even worse with a front running from the Channel to the Mediterranean without a break, while the CPs welcome an enthusiastic new ally.
I won't deny that in a CP Italy scenario, esp. if the CPs take a defensive stance in the West, France is going to be sorely tempted to violate the neutrality of Belgium, Switzerland, or both. However, this is most likely going to fail, and make the Entente problems only worse in the long term.
the french has at least 3 in-roads:
1) the alps: ok, it's difficoult but naopleon managed it
He also managed to romp through Germany with little difficulty. Noticed France in 1870 and 1914 getting any luck about it ?
2) the coast road: much easier, starting from nice and going up to genova
Italian artillery can play the turkey shoot with advancing French infantry.
3) landings: against the french Navy, the italian one would not have a chance, expecially since 2/3 would be in Taranto and only 1/3 (maybe 1/2) could be harboured in genova-la spezia-livorno.
It takes more to pull a successful landing than naval superiority. In WWII Allied landings in Italy almost failed, in WWI the Entente would have no technological or force superiority against the defenders, and no help from the air.
Also, think about historical suggestions: Napoleon won a couple of wars against Austria invading northern Italy. would not that appeal to froggies?
Oh, sure, just like they expected they could break through Alsace-Lorraine and Rhineland if they tried with enough enthusiasm, because it had been done by Nappy.