I find this fascinating, forensic analysis with limited data ! A couple of points, the chart link provided by Glyn above does show limited visibility, thats not to say there couldn't have been a clearer area in the region but the odds would be against that, so a sighting would likely be closer rather than further away. The possibility of a mirage are still there. There is an inversion ceiling on the Long Beach ascent at between 600m and 1056m which could produce a mirage although I am not ofay with how much difference in density is required for this effect. I must say I am a believer that MOST UFO sitings can be explained by natural phenomona.
As Nigel has indicated the difference between th DP and Temp is quite substantial, in the region of 13 to 20c except at the 650hpa level on the Long Beach ascent where it is only 4.9c, so it wouldn't require a huge drop in temp to produce saturation point.I would guess it would require a vertical displacement of about 600m (should be able to work it out but my brain isn't functioning properly at the moment !). Nigel is far more knowledgable in this area than me, perhaps he could say if my reasoning is reasonable !
Cheers.
Paul D