Resistance and impedance are not unrelated, but only impedance matters when it comes to measuring speaker performance or designing crossover.
You can't expect to calculate impedance correctly. You have to measure it with a frequency sweep, and you have to do that measurement while the woofers are mounted in the cabinet. The cabinet design affects the low frequencies of a woofer. (The impedance curve that SEAS published doesn't show cabinet effects.) That's why that D'Appolito article had so many impedance graphs. He designed both the cabinet and the crossover.
You can't properly design a passive crossover without knowing the impedance of the woofers and tweeter in the frequency range affected by the crossover. I believe the Thor MTM has 3rd order crossover slopes, so that would be 2,500 Hz ±1.5 octaves, roughly 1,000 to 6,250 Hz. If it has 4th order slopes, that range would be narrower, ±1 octave, roughly 1,250 to 5,000 Hz.
For those metal woofers, don't even think about using 2nd or 1st order crossover slopes. You'll hear ugly sounding break up noise from the woofers. See the W18's large break up peak between 3 and 6 kHz. You'll also hear distortion from the tweeter. That distortion might be short lived, as the tweeter might fail.