Btw, if place a rectifier diode between +12 V and point A on my picture to cut the negative halfwave, would that be any help for LEDs (they would not be exposed to reverse voltage)? Price of a voltage drop due to diode forward voltage is acceptable. Do I miss some other drawbacks?
Other than the increased part count and voltage drop, both of which seem accecptable to you, there really aren't drawbacks to adding a rectifing diode for this type of application. It would be nice to find-out exactly
what's causing the ~5 VAC
sinusoid, but that might not be practical.