The resistor and diode ground the circuit through D+ when cranking. This gives you some back up bootstrap current in case the charging bulb blows, as well as giving you “bulb verification” at start up. Once the alternator is charging, D+ goes high and no longer serves as a ground. If the alternator fails, D+ can become found and light the bulb. But it wouldn’t be the only bulb lit.

The switch is at the bottom of the sender as mentioned. What I would do is identify which of the three sender wires feeds the reserve light, and just disconnect that. If the problem goes away, it’s in the sender, otherwise it’s a shorted wire.