I just went back through this thread and I can't find the "focal" length of your camera. But I'd think .39 would be about perfect if optimal is 0.42 for film. Flare can be a problem ( or a feature!! ) any time the sun hits the pinhole directly. I do not think the distance the paper moved from having 3 papers loaded could matter. Maybe there was some light leak around the papers after all?
I ignore reciprocity failure in paper negatives, but have a suspicion it is there in very long exposures. The few times I have made exposures over 10 minutes, the effect has not been as strong as I expected, and I've noticed some strange things that might be different speeds in the different parts of the VC emulsion when the exposure gets over about 10 minutes.
I too would like to know if other people think about reciprocity failure with paper. Or if anyone has noticed this "color separation effect" between greens and blues in long VC paper exposures.