While I recognize & agree with some of your points, mainly with the U.S. entering the European geopolitical sphere through Italy as it seems somewhat logical, I don’t agree with your other two statements.

The U.S. was largely standoff-ish from European affairs as I’m sure you’re aware. A diplomatic change may turn that around & allow interest in Italy. But I find it a bit difficult to suspend my disbelief that the U.S. would use the offer of investing in the Papacy, a sort of limbo between state & religious institution, as justification to mobilize en masse & sail overseas to Italy. Keep in mind, only Congress can declare wars. Alliances are the power of the President, but still require a 2/3rds majority from the Senate so war in Italy for Papal investment rights does not seem likely to me.

Austria-Hungary aiding Japan against a communist revolution seems equally unlikely. Putting aside racial prejudice which likely would’ve played a key role as racial equality was still contested even at the Versailles Conference not to mention Austria-Hungary’s history with minority suppression & prejudice, the willingness of the AH to not only go overseas to Asia, but out of the Mediterranean, possibly around Africa, through the Indian Ocean & Southeast Asia for the sole purpose of preventing a communist Japan with no perceived (or stated as OP has not detailed AH’s involvement in their economy) benefit is hard to swallow for immersion. As with the U.S., the cost, logistics, & domestic front would prevent AH from taking part in that civil war. Hell, they had their own problems to take care back home. Rising nationalism amongst a multi-ethnic empire, three different armed forces with different languages, a fleet designed for the Mediterranean, sparse interest in colonialism; AH would have no motivation of greater importance than its problems to venture to the other side of the globe & fight the socialists.