Hum, I've been struggling to seal the deal on the corrupt via infiltration strategy.
I think the time or two I've managed a plain enshadow via diaspora type win, I did it primarily through refugees and
intentionally getting the cities of my enthralled lords conquered, preferably faster* than anything can manage to suss out the lord was the problem... and even if it isn't, after much of the lower nobility has been fairly well shadowed. If you can basically get the shadow chain to self-propagate, you're in good shape. Making small nations into what amounts to corruption bombs for larger ones, as they conquer cities and integrate parts of the leadership, seems pretty effective. You don't so much directly infiltrate as lace the world with hotspots and let things spread from there, and the fleeing refugees that spill out while a particular city is heating up just helps things (especially if you're using tranquility or dribblers, from what I've noticed).
... I guess basically, what I'd say try is, instead of stopping the small nations from banding together, take some time before you start in on the big one to make
them a source of corruption. Hard burn sequential enthralls (getting them killed before moving on, to keep costs down) to make yourself a mess of messes. If they're spreading shadow, it doesn't really matter if they've banded together and are taking chunks out of your nation, 'cause that's still a net win for you
*There was one fun bit where I had a single lord end up getting like three separate nations destroyed/conquered, by repeatedly ruining the city they were in and being subjugated, the promptly working their way into another leadership position and doing it again. People knew the guy was working for the darkness, even, but their nations didn't tend to last long enough to vote for an execution, or were small enough the enthralled's vote could split the execution decision :V
A continent or two ended up getting mostly burned down trying to kill that guy. Was kinda' hilarious.
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... though I do have to admit, I've used the interpersonal/political stuff basically never, usually just focusing on enthralling/enshadowing one or two city lords and then beelining for population conversion. After that, the passive effects of that seem to work fairly effectively -- if you aim straight at it, even if you spike evidence through the roof and get the entire world to jump you, the damage will already be done by the time the armies arrive. Oddly enough, if fleeing refugees and conquered populations aren't exactly human, the shadow seems to have a relatively easy job of spreading, heh. Or in other words, infiltrating an already compromised population is a lot less trouble.