its never successfully only using 1 layer with snap fades and movie files - you can only really get away with it using still images - even then, you can still get a 'flash' before the image is displayed, especially if its VERY large. Your best option is really to program it as an 'A/B roll' as you put it.

However, if you are still using a 0sec fade, depending upin the desk you are using, you may have to delay the intensity by 0.1 of a sec to allow the clip to load before you 'snap' to it.... this avoids 'flashes' when doing 0sec x-fades - also make sure you put the 'hidden' layer to zero - or none of the above will be effective.....

S