At a typical water usage of 60 gpd/person, a 1 cu ft resin tank will regenerate every 6-7 days when regenerated with 6 lb of salt.

Do they have any high-water-use fixtures, such as a jetted tub or multi-head shower?

I would not, under any circumstances, get the resin/carbon tank, whether the media is mixed or in a split tank configuration. It forces too many compromises: 1) The resin portion of the tank won't be tall enough to allow the right amount of expansion during regeneration, 2) the carbon portion will be too shallow to do its job - you need 2-3 ft of depth, 3) The flow rate for backflushing the carbon adequately is higher than that which is acceptable for the resin, 4) carbon beds should be backwashed daily, whereas resin regens happen weekly, so the carbon bed will become compacted and damaged, 5) that damaged carbon media will require replacement very quickly.

If they really need all of the chlorine removed from their water, a separate properly-designed carbon system is the correct way to go about it. However, be certain they have just chlorine and not chloramines or a carbon system will do little good.

No special high water use fixtures (just regular shower).

Do they even need the carbon at all especially if they're planning to get a separate system for drinking water?

Whew! Thanks that answers one big question and helps eliminate some unnecessary options!

Any advice on how to go about evaluating regeneration monitoring/efficiency? Is the water $$ and environmental savings worth spending more for a system that can monitor real-world usage and only trigger regen when necessary? Are there significant differences in implementation of such systems that I would need to watch out for?

Any modern electronic controller will regen based on the gallons of water used, with an override to ensure it doesn't go too long between regens. Of those, Fleck and Clack have the best build quality.

There are two schools of thought on regen: 1) Add water to the salt a couple of hours before regeneration, 2) Add water to the salt immediately after a regen. Of those, #2 is the better option in nearly all cases as it takes ~48 hours to get a good, strong brine solution for regeneration. If, however, you are in an extremely humid area with wild temperature swings between day and night, you might consider #1.

This is very helpful. We found two local water treatment specialists that mention Fleck or Clack on their webpages (and not one of the name brands like Kinetico). We'll contact them this week and update with what specs/systems they recommend!