Electrical – do multiple neutrals and potentially make a bad situation worse

No way from here to know which wire fed the light or fan, but you have a continuous hot feed into the box that fed the receptacle, light and fan. The third wire, maybe it fed the fan or maybe something something else farther down the line, if the latter that needs to be tied in 100 percent to the hot feed. All whites still tied together, blacks to the dimmer from the light and hot. All grounds tied together set with a screw to the metal box. Securing the ground to the box is not needed if the box is plastic.

Before I got too wordy, here is a sketch

Still have to learn to format better

The sketch is a little rough, it should get the idea across I hope

I believe you are correct in assuming that one switch controlled the fan and one controlled the lights originally.

My guess is that the wiring at the fan just has both the red and black wires connected to the line side of the wireless receiver, which would let either switch turn the fan/light on.

I would look at the wiring at the fan to confirm this. If I am correct, then yes, you should be able to disconnect the red wire. I would disconnect it from the fan and from the switch in the box and cap both ends.