Bevel, then spile (duh)
I spiled and glued-up plank 5 today, and came to a realization that I should have figured out earlier. For previous planks, I have beveled the landing for the next plank just before hanging it. I realize now, I should have beveled the landing prior to spiling. So doing brings the top batten plane (the plane of the last plank landing) into closer alignment with the plane of the bottom batten of the pattern (the plane of the next landing). This results in a spiling pattern that is significantly less twisted, which seems to result in a plank that is less distorted.
It may be that I have overestimated the impact of this change, as this plank (the 5th of 8 pairs) would have been easier anyway, but it is nonetheless clear that this is the right way to do it.
The plank went on more easily than any previous.
About 4 hours to spile, fit, cut gains, and glue up the pair.