There was a Ranch to begin with. This is usually how quests work - the quest locations are already set in the world, the Quest in particular just seems to choose the closest location acceptable for the quest and puts, if necessary, monsters or npcs there.
Nope! Basically, if there’s a Ranch, they have a chance to settle there, that’s about it. The Ranch itself doesn’t seem to have any specific mapgen values either which would stop it from showing up near problematic stuff either, so more often than not it’s a shot in the dark if the Outpost will ever be able to survive.
From what I know, Stuff only ever happens when it’s inside your reality bubble. Everything outside is unloaded and not considered to be moving or doing stuff at all.
This should apply to Fungus as well, meaning, they do not spread the Fungus unless you’re close enough. Of course, your reality bubble is reasonably quick, so even just napping somewhere could produce a field of Fungus closeby which in turn can spread when you do other stuff nearby.
The Fungal Creatures spawn semi-randomly around their starting areas (bloom, tower…). They seem to have quite the reach though, considering I found fungal creatures quite a few maptiles away from any place they could have come from.