Cable-to-cable crosstalk (Capacitative Coupling)
Source logic most likely means that your source is some digital logic circuit, (some microcontroller or something) which has standard CMOS voltage level of 3.3V. Explained here. As you cans see, logical "zero" is represented with any voltage from 0 to 0.8V and there is no danger that your coupled noise (0.4V) induces something that may be considered as logical "one" (which is represented with voltage higher than 2V).

Regarding your -26.4 dB question: look at this: Cv is same as Ccv as the lengths and the medium are the same. Input impedance is 100 Ohm, way smaller than any impedance 1/(omega Cv)=2kOhm . Practically, 100 Ohm in parallel with 1/(omega Cv) will be 100 Ohm. So, if you calculate your voltage divider 100/(2000 + 100) = 0.047 = -26.4 dB!