a) Cross-talk pick-up which can be minimised by inter-core grounding or low impedance loading of the cores
b) degradation of the high frequency components on the leading and trailing edge of the pulse due to cable capacitance
It really is irrelevant whether you are transmitting 1 pulse per day or 10,000 per second; the above problems still exist (or not, if you aren't too bothered about cross-talk or pulse shape)
Um, not it isn't.
Crosstalk is caused by changes in the magnetic field of neighboring wires. How efficient do you think a transformer would be if the input was 0.00001Hz (compared to, eg. 10,000Hz).
If you're only transmitting one pulse per day, do you really think it matters if the leading edge is perfectly square or not?
And ... if you were worried by crosstalk (which you're not), the simplest solution would be to separate the wires of the ribbon cable.