Rotors and diode boards are kind of hard to test statically since under load they may not stand up. They may test good statically, but act oddly in use - seem to be working, but not quite right, or not work at all. The rotor windings may fling outward and short, the diode may function with the meter current flowing through it but not with actual power from the alternator at operating temps.
One of the places to look for bad connections (and quite often overlooked) is the light board in the gauge cluster. Two out of three I see have high resistance and my handling is to solder the foil to the input pins.
I doubt the high beam indicator blue lens melted due to too high wattage bulb. There's a long tunnel from the bulb that channels the light to it, and besides, those sockets will take only one size bulb. It's more likely the lens deformed over time from the sun and getting pretty hot in there.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*