A WD is supported against gravity by degenerate electrons. Pressure from these electrons is independent of temperature. In describing 'visual binary' make it clear that two stars are visible. On the other hand, the binary will be seen as a spectroscopic binary at all tilts of the orbits to the sightline except the perpendicular one. Clearly, the probability is much higher that a binary will be spectroscopic than eclipsing.
All eclipsing binaries are also spectroscopic binaries.
But their orbital motions will still be detectable through the Doppler shifts of absorption lines. Except for the fact that the brightness of the stars decreases with increasing distance, detectability of spectroscopic binaries is in independent of distance. And there will be NO white dwarfs in the young cluster. Therefore, as T increases from base to tip, the R must decrease. A simple way to show the lack of a L-M relation is to plot the evolutionary tracks.