Circuit and components for DC Fan

PWM is used because it wastes much less heat than "linear" methods like that shown in your schematic. A TO-92 voltage regulator and a DIL8 microcontroller is all you need to read the voltage from a 10kohm pot and then generate a PWM signal to put through a power MOSFET on the low side of the fans. You could even substitute a 555 and associated passives, but the MCU will give you better control.

^{simulate this circuit – Schematic created using CircuitLab}

The solution is not to use the wall switch to control the fan, but the remote, as the product is designed; and to produce a wall holder for the remote which is placed such that people coming into the room are guided to use the remote rather than the switch, and such that they can invoke the "light on" function of the remote without removing from the holder, so that the remote acts as the de facto wall-mounted light switch for the room.

The issue is clearly that when you walk into the room, the main switch is always in its expected location, which reinforces the behavior of reaching for it, whereas you have to look for the remote. If the remote is in a holder that is more conveniently reachable than the switch and as easy to operate, that problem will solve itself via user behavior modification.

You could also make modifications to the wall switch to make it less convenient to operate. For instance, if it is a large, flat rocker switch, build a cover for it so that the rocker can only be operated through finger-sized holes in the cover, discouraging regular use.

If these Hunter Fan people were more clever, they would sell or include junction box covers that double as remote control holders. That way users could uninstall their light switch (wiring the circuit so that it is permanently on) and replace the switch panel with the remote control holder panel, so that the remote control now appears exactly where the switch used to be.