IDK, maybe you treat all true neutrals as independent nations (no blocks, or alliances w/other powers or neutrals). Mongolian tt aren’t included, they remain part of the NAP.
Pay 5 IPCs to roll a diplo dice. You can buy as many as you want (or maybe limit it to 1-2 per turn), but each one is dedicated to only one true neutral tt per turn, and you must be able to invade. State what true neutral nation you are trying to sway/invade (must have units dedicated to the invasion, and can’t make changes after dice is rolled). Once you start the process there is no turning back, you must follow through that turn.
Roll your diplo dice.
Roll 1 tt becomes pro your side (don’t have to fight), and the standing army joins you (immediately becomes your color) because you negotiated them to join your cause (you get to include any tt IPCs in your income).
Roll 2 tt becomes pro your side (don’t have to fight), but the standing army doesn’t join you (it is dissolved) because you negotiated a peaceful occupation, and they laid down their arms (you get to include any tt IPCs in your income).
Roll 3-4 tt becomes pro your enemy. You have to fight the standing army at least one round of battle and if you retreat any surviving units immediately become the color of your enemy (enemies choice of controlling power). Whom ever controls the tt will get the IPC value in their collect income phase.
Roll 5 and the standing army is doubled, you must again fight at least one round of battle, but then can retreat. You under estimated the resolve of the country and their will to fight. Any survivors from the standing army would immediately become the color of your enemy (enemies choice).
Roll 6 failed attempt. You lost your investment, and it was deemed not in the best interest to continue negotiations or attempt an invasion. This was a diplomatic process, so nothing changes, and the standing army stays intact.
As note #1 I also think that any neutral tt w/o standing army, or IPC value should be awarded 1 inf (maybe 2) so the US can’t just waltz down through say Columbia w/o resistance to get to the more valuable S American nations. All the above would apply.
*As note#2 you could beef up the 3 major true neutrals to include better units in the standing army.