Well, this will require some pondering! As Brian says, the biggest obstacles are political instincts (not to mention the jingoistic attitudes that fanned the flames to begin with) ...
One notion occurs about the US and the League of Nations. Suppose Woodrow Wilson or advisors anticipates the mood in the Senate. Full LoN membership was a threat to some Senators (mainly Southerners, IIRC). What if the vote was on options? (I) Full Membership; (II) 'Observer Status' (as a 'founding member'); (III) No Involvement with LoN.
Historically, we know that (I) was rejected in the Senate. (II) would allow DC to keep tabs on those pesky Europeans - without US sovereignty being infringed. (III) ensures US sovereignty remains uncompromised. But isolationism also means almost zero input into international affairs.
The big unknown is, should (II) prevail, would American policymakers see advantages in taking on full membership? Or would it prove to be just a slower route to US isolationism?
Marshall Plan: There was self-interest in the Marshall Plan (thwarting the growth of 'domestic' communism in Western Europe, maintaining trade and a global market for US goods, etc.). But the Marshall Plan also represents astonishing generosity. The mood of US citizens and policymakers in 1919 was a lot more insular (and hard-nosed). But, OTOH, the US was far less militarily dominant after WW1 (compared with WW2). So how, for example, did US policymakers intend to back up debt-collection?
In a sense, the US was relying upon Europe's central bankers to honour their wartime debt repayment promises. But what if those central bankers had argued to boycott those debts to force what's now dubbed a 'haircut'? Could the governors of the Bank of England and Banque de France have forced the hand of their American creditors during the Depression of 1920–21? If so, renegotiated loan payments could have freed up funds to rebuild European shattered economies, provided jobs for returning veterans, etc. In other words, US largesse à la the Marshall Plan may not have been necessary.
Veterans: A lot of returning veterans faced unemployment. I'm wondering about international military units in the name of the League of Nations. It would have to be toothier than UN peacekeeping - eg: no invite required from the offending nation state(s). For manning, perhaps some formula for set percentages of troops whose origins were in former Entente Powers, Central Power, and neutral powers?