Was going by the game text rather than the fiction but I'm not sure either of these change that. Both a safe and a faraday cage are standalone structures. They're not (usually) part of the building containing them.
The 'Faraday Cage' was a metaphor used to describe the effect of the wards Harry placed on the facility, not the structure the wards themselves were built upon.
The wards on that facility were explicitly described as being composed of numerous weak protection spells layered one upon another until the whole created the desired effect.
The quote from YS about being cast 'all as one spell' supports at best a tenuous conclusion against what I described; just as easily, perhaps more so, it could be read to require that any and all landmines attached to a
particular ward must be cast as part of the same spell as
that ward, and placing no restrictions whatsoever on whether other wards, with other landmines, can be supported on the same threshold.