How about this for a solution: scale the percentage chance of an opposing power's diplo chit firing with the % that the power is already in the war, while keeping their full counter-diplo ability?
So if USSR is at 25% mobilisation and Germany has two 5%* chits in, Germany has a 0.25x0.1 = 0.025 (or 2.5%) chance of scoring a 7*-10*% hit, pushing the USSR that much away from the Allies.
If UK has 1 5% chit invested and Germany has two, Germany now has a 0.25x(0.1-0.05) = 0.0125 (or 1.25%) chance of scoring that same 7-10% change.
If UK now invests 3 chits, two cancel out Germany's (which both count as -5%) and the one left over has the full 5% chance of firing to swing the USSR to the Allies, partly to ensure that majors can't get locked at 0 mobilisation forever and also because Stalin is more likely to listen to the foreign ministers of people he is likely to side with than those he would side against.
But if USSR mobilisation is now 80%, those 2 German chits are now much more effective (8% vs 2.5%), which allows them to delay USSR's entry to the war (which is the entire point of diplo chits in the first place) without adversely affecting MPP too significantly. But at 80%, the USSR is already close enough to entering the war that they won't be delayed indefinitely or crippled by a couple of pro-Axis hits.
The same would apply to all other majors too (so UK chits are less effective against Italy) but not minors (the system doesn't make much sense if a minor is at 0% either side; in major power terms this means they have no interest in entering the war, but for a minor it just means they are unconcerned which side they might join). None of the minor powers are strong enough to really make a diplo-spam strategy worthwhile, if someone attempts one on say Spain, whatever benefit they gain from the Spanish forces is approximately equal to what benefit they would gain if they invested in something else, rather than being disproportionate the way an 800 MPP investment in the USSR is.
* = I'm using the SC2 numbers as I'm much more familiar with that game than SC3, but the same concept applies either way.
< Message edited by BiteNibbleChomp -- 8/30/2018 6:57:32 AM >