Yes, you are correct, I can't lower the base parameters in the salt mix, just raise them as needed, however I've not seen too high Alk or Ca using Instant Ocean, my Alk runs 8.6, Ca runs 450 - 500 , both after dosing.
The skimmers draw from a separate sump where there is a float switch that powers the fresh seawater reservoir pump, so as skimmate is removed it's replaced with fresh seawater, or dirtiest water is replaced with cleanest water.
If skimmers go nuts and drain the sump there is another float switch that turns off the skimmer pumps. In the event this happens there is enough water to allow circulation through the sumps and run the return pump.