That also means if you can hear some of the crosstalk, it'll be like IMD - unmusical - ugly - my guess is "grainy". You may say that's not audible, and I'd agree, but the crosstalk is different in phase, not just amplitude AND it's imbedded throughout the adjacent passband, not just the skirts, even if it's not apparent in the trace. My measurements thus far show the skirts of mid and high pass passive networks beginning to fall apart -30dB to -50dB below the passband primarily due to poorly placed low pass inductor(s). Mutual inductance between (air core especially) inductors can be a big deal when modulated by audio. If you mean by "mutual inductance" the presence of nearby ferrous material or unmodulated magnetic fields (driver magnets), you're probably right and it's on the dog. How bad from an audibility point of view will vary, but I bet that input cup mounted three way crossover in my Heresy II's screws up the mids audibly. I disagree with you about mutual inductance being purposeful, 'cause it's bad. Why fill the holes? I think you nailed it - to keep the plates from shifting.
They used ERSE 14awg IXQ steel plate cores that add mounting holes through the plates at either end, but they mounted them on a bed of silicone glue with wire ties and filled those holes. You just answered a question I've had about some Danley passive crossovers I replaced with my own a few years ago. Cranky soldermeister is right! At least the first part - I'd have to see some of your solder work to opine on the other (meister is German for master) : )