Both crimping and IDC turned out to be too much of pain and time sink for me, so I settled on what I think is the easier and faster and does not require either. It does require a lot of twisting, and I'm investigating if I can reduce that, but otherwise, I'll live with it.
I bought a bunch of pre-crimped cables at Pololu of all the various, appropriate lengths I needed. Available here
I also bought a bunch of connector housings in one and two rows. The former for the sensorboards, the latter for the ADC boards. The number of positions also were strategically chosen so that one final cable assembly connects two sensorboards (hammer and key) to its ADC board on a staggered mode: hammer-key-hammer-key-etc
Twisting the 12" cables which go to the sensorboards at the key is a bit tricky and I am still perfecting the technique, but I'm improving. One key to the success of the "perfect twist" is to not only twist the pair around each other, but to twist each cable on its own axis too. When done properly, it results in a fast, stress-free, easy-to-bend pair, such as the orange gray on the picture below. If not done at all, it will result in a holy mess which will want to spring back in its untwisted position. When done so-so it will result in something that wants to make loops, has some internal stress, and is somewhat difficult to put in the position one may want, like the red-black and blue-yellow long pairs in the picture below. Note that the short pairs are much easier and forgiving to twist.
Probably it's clear and does not require a picture, but here is the detail of the 2-row ADC connector and its nearby connector for the sensorboard going to the hammer
And here is how I plan to assemble them, for now with a paper mock of the actual ADC boards:
Building a cable serving 4 notes, like the one shown in the above pictures, is comprised of 16 wires: 2 sensorboards (hammers and keys) each with 4 sensors; each sensor has two wires. It took me slightly more than 30 minutes, 15 of which twisting the 12-inch wires, and 10 minutes twisting the short cables (in the picture shown is 2 inches, but in other situation I will need 1 or 3 inches). It took me total 10 minutes to put the connectors inside their own housing: when the cable is twisted it's trickier than I thought, and again there are 16 wires and each has two termination, so it's 32 insertions.
If I don't improve my skills, that will take me 11 hours of work, which is reasonable but on the "I wish this was faster" side. I suspect I could reduce the time by 30-50% by improving my skills, but again that's still about 6h of twisting, which is too much. I am thinking of a LEGO-Technic basic jig to allow me to twist the cables of the right amount (on each own axis and around one another in the pair as I mentioned above). I will report here if I get any success at that.