Interesting. I like the idea that an invitation is required to enter, but for me I imagine less sentience and more instinctive biology. I think it likely that until they learn that a specific type of barrier (e.g., glass) can be broken and bypassed it is probably seen as impassable.
Certainly a lot of glass in office buildings and expensive homes is reinforced somehow nowadays, so a few early BO's failing to break fancy glass (tempered, embedded wire mesh, film-coated) may have caused a mistaken impression that all glass is impassable and not worth trying to break - especially since for BO's vision is not their primary sense and they can't see through it.
Witches seem to sense/remember that it is different somehow, and so do the Grendels. If they (W/G) are different/higher levels of complexity in the ecosystem or within a larger single organism connected by the fog (that somehow transmits information between parts), they could still be relying on that mistaken initial assumption...but they will probably learn very soon (likely by accident) that at least some glass is easily broken, and that will immediately put the traveling human group in much more danger because safe places to stop and rest will suddenly become limited to only locations wtih reinforced glass. (to be safe from Witches) and who knows where to avoid Grendels, who could clearly smash through even reinforced glass. Cheap subdivisions where they are now are a likely place for W/G's to figure this out, since cheap subdivisions mostly use cheaper glass...
I also see no issues with power lines being unaffected (yet) by corrosion given that they are underground and heavily shielded with corrosion-resistant casing. The water starting to taste funny could be lack of QC at the source (as speculated by one of the group) OR an indication that the corrosion is starting to break through the thicker metal of water pipes, which are thicker but not shielded in any way. I wouldn't be surprised if a member of our group gets turned because they drink too much contaminated water that the others drink more sparingly because it tastes bad.
I'm not sure where they idea of the corrosion of electronics being "selective" is coming from. Maybe I missed evidence of that but I don't remember anything that seemed inconsistent.
Anyway, I like supernatural stuff, but personally I hope this turns out to be natural because I think that makes for a better (and more horrifying) story personally.
