FMC defaults to 240/FL100 for ONE reason: VNAV will NEVER exceed 250kts below FL100!
VNAV allows for +10kts faster to be flown, thus defaulting to 240 ensures compliance.
Conversely, the A/T "wakes up" when VNAV gets -15kts below commanded VNAV speed.
Use the systems as they are designed, in parallel, and we end up with an aircraft idle descent not exceeding limitation...
Regarding CI, as mentioned in this post:
1) CI for descent is linear between KIAS and CI, hence low CI relates to low KIAS, 6 being 245, 0 being optimum glide for fuel at 239.
2) CI for CRZ and CLIMB take into account many more parameters such as weight, level, toc wind (calculating a speed correction for the calculated hw/tw component 150nm along the track direction after takeoff by using the angular difference between the track and toc wind direction), toc isa, to name a few...