Soil is my favorite thriller in any media full stop and I will have to say that weirdly enough, a lot of Ura Baito's gray activity would not fall into the foreign bodies category. A lot of the gray I remember actually seemed to be following and upholding some kind of law, code, or order which would be the complete antithesis of Soil's foreign bodies from the path to the curse-breaking temple, the last Kuroiwa job, and hell even that god-like guy who resets things to right wrongs in the timeline (who we found out later has been MIA for a while). The only one I can remember that I can 100% say fits the bill is the quantum residue of that actor that kills anyone who tries to observe it.
The way I understand it, foreign bodies include anything that should not
normally exist in reality, but if for some reason too many of them gather in one spot, then the connection between reality and irreality becomes stronger and stronger that eventually the border will break, making it much easier for irreality to spill over into reality. It is implied throughout the manga that even manmade objects intentionally built to be "out of this world" can count as a foreign body. Certain acts that are so bizarre you think it's "unbelievable" also count, I think. As more and more foreign bodies gather/happen in one place, they in turn invite even more foreign bodies until the boundary between reality and irreality breaks. However, once reality becomes folded too far into itself, it rubberbands violently, kicking out all foreign bodies and snapping everything back to
normal or at least something approaching that state. I am looking at it from this angle. I am not sure which chapter, but in the Ura Baito universe, I think it was mentioned grays can't just "invade" our world willy nilly - like you said, there are "conditions" that have to be fulfilled for a gray phenomenon to take place. This is the root of my belief that the power of grays in the Ura Baito universe is directly correlated with how you perceive them and whether you comprehend them.
In my older comment above I referred to Daidai's condition as the inability to comprehend, or even perhaps an active refusal to comprehend. Since Daidai does not acknowledge a gray as something that is
abnormal, they in turn seem to have absolutely no
real power over her. What made me think of Soil is this aspect; I'm not saying that grays are identical to foreign bodies. In Soil, everyone thought the Suzushiros were real people (basically everyone refuses to see them as foreign bodies), and so they continued inviting more foreign bodies into the town, and so on. So, the act of observation and comprehension do not really come into play with foreign bodies; however, there are still quite describable rules; including how the infestation even started in the first place. There was a "patient zero" in Soil, which the very last page confirms to be Ms. Kosaka (and her actions).
What if there is also a "patient zero" in the Ura Baito universe?