By design, the 4.3" is capable of using either unit of measure. One thing that hasn't been asked - what is firmware version of the instrument in question? _02182019 is the latest revision level.
I now understand why you are seeing the responses you are reporting. The instrument is performing in the fashion it was designed. The problem arises in that your installation is not in an anticipated configuration.
I don't believe you have disclosed which model of the concealed side mount control is in use on your boat. Based upon the method used to obtain fuel level (Navico insect), I believe you have the newer basic concealed side mount. One difference between the basic and full-featured controls is the latter provides fluid level sender inputs which directly accept a resistive sender element. The basic control does not offer fluid level sender inputs.
The 4.3" instrument is anticipating an instance 4 fluid level message from the control head. As a result, the installed and configured as instance 4 Navico insect is not actually communicating with the instrument. This is why you are not successful in altering the tank volume using the instrument. In fact, none of the cal/config functions for this fluid instance will work via the instrument. (Level calibration, fluid type, tank volume).
If you have the full featured control head, you should remove the Navico insect and connect the sender directly to the wire pair in the control head harness labeled fluid level 0 - tank 1. Doing so will allow full control from the instrument.
If you indeed have the basic control head, you will need to have the Dealer alter your tank capacity using EvDiag 6 diagnostic tool. This will then provide accurate data to the instrument but you will still be unable to change configuration with the instrument.
There may be a future firmware update for the instrument which will eliminate the problem you have experienced. In the interim, EvDiag 6 will get your install providing the accurate data you are after.
I have duplicated your setup using an insect configured as instance 4 for fuel level data. As long as there is SOG data from a GPS device and the EMM is reporting valid fuel flow data, the ECO page displays accurate estimated range information.