Assuming that the 2018 i3 works like earlier models in these respects, this article
should answer some of your questions. Unfortunately, it refers to the iRemote app that has been deprecated in the U.S. in favor of the BMW Connected app which has a different user interface and somewhat different functionality, so references to iRemote in this article don't always apply to BMW Connected.
Our i3 doesn't have the heated seat and battery pack heater option because these capabilities aren't needed in Honolulu. I have never enabled battery pack preconditioning to cool our battery pack because it is never hot enough where our i3 parks overnight to make preconditioning useful. I have never used the low-cost charging option because it has been very buggy in various i3 system software releases, so I have used the reliable charging start and stop time functionality of our EVSE instead.
Setting a departure time is required for cabin and battery pack preconditioning and for low-cost charging. It is understandable why this would be true for preconditioning, but less understandable for low-cost charging. Apparently, the i3's system software tries to guarantee a full charge by the set departure time, so it will start charging prior to the start time of low-cost charging if necessary.
Also, even with low-cost charging enabled, charging would start immediately if the battery pack's charge level is "too low". If this occurs, the proper behavior would be to charge only to a "high enough" charge level at which point charging would stop until the start of the low-cost charging period or until the time required to ensure a full charge by the set departure time. Buggy implementations have prevented this from working as designed.
As one can imagine, there are many opportunities for software bugs to result in incorrect behavior. BMW has attempted to make the charging behavior so foolproof that it is difficult to understand in addition to implementing a user interface that isn't as good as it could be.
Preconditioning and low-cost charging can be controlled separately, so you should be able to enable low-cost charging without enabling either cabin or battery pack preconditioning.