As for the mountain waves. See above. If you are bashing along in a C172 and Best Rate is say 68 kts, but 65 gave you an extra 100 fpm RoC, you would be a fool in my opinion to 'PEG' the 68 kts.
Monopole, that doesn't make any sense (much like the red line comment). If you have a best rate speed of 68 knots (for given conditions/weight) you will not get extra ROC an ANY other speed. The problem you're eluding to, is not knowing exactly what your Vy is, and yes, you could work it out by experiment, but you should have a good idea given the Vy at MTOW, and the current aircraft weight. In the example you've given, 65 kts IS the Vy speed for your 172 given the conditions - by deffinition (even though 68 kts may be the MTOW Vy given in the flight manual).
Pilots should know the approx speeds to fly at different weights. If you ask me, it is stupid NOT to follow what you have been taught, and practiced in an engine failure. The stress of an urgent situation is no time to become a test pilot. By all means, do it under simulated conditions, that is good airmanship.
Close to the ground, you would normally be best to use Vxse (normally 5-10% below blue line) after an engine failure in a light twin, that is basically the proceedure in larger aircraft. The danger is, as you reduce below Vxse, induced drag increases rapidly, and approaching red line, you start to run out of options.