Although I can sort out phased sinusoidal fading, with two led's 180 degrees out of phase, when one is on the other is off, how do I do I a sinusoidal fade up or fade down without cycling?

What is your definition of a sinusoidal fading? Because a sine wave has a negative portion. Do you perhaps mean a sine superimposed on a fixed value, like half intensity?

If so, your answer lies very simply in the start and end points of a half wave which are at -pi/2 and +pi/2 (-90 and +90 degrees). Or something like that.

What I mean is non linear fading along a sinusoidal waveform with the wave lifted so that the lowest value of the negative portion of the wave is 0. So I want to fade from max to min or min to max to give a gradual on phase and a gradual off phase to the sequence start and end sections.