Good analysis!
Although...
But the funding increase overall, like I said, is effectively proportional to the score, sans corner cases.
This is not really true in either the strict sense (proportional = constant ratio) nor the weak one ('strongly related').
First, since there are a lot of countries and only so many aliens/cultists/shady orgs to go around, alien activity is going to be zero or near zero for most of them. If you don't believe me, go and check a 1999 save for 'activityAlien'. You'll see a
lot of zeroes.
And once you clear the 'good > bad + 30' threshold (300 score as a minimum, probably 500 to be safe), you're likely to get an increase from most of them, barring local score spikes.
Second, the score increase is
far from proportional. If you have 2000 score and the aliens scored 100 in the country, that's 200 vs 105, a rougly 50% chance, as you said. If you increase your score to 4000, it's ~75% chance, a ~50% increase in funding for a 100% increase in score. Going to 8000 is 200% increase for ~75% better funding, 16k is 400% to 88%, and so on. These are pretty strongly diminishing returns, and that's one of the countries where score actually matters. For most of them, 1000 score would already make the funding increase a near -certainty (100 'good' vs a few points of 'bad').
TLDR: What matters more is not big score numbers (unless you use 'performanceBonusFactor'), but rather offsetting alien activity with your own.
Kinda wondering which is worse, to ignore a mission or to get wrecked there.
Ignoring the mission, since alien score is vastly worse for the country than generic negative score.
Incidentally, that also makes slander missions a real menace to your funding.