Secondly, the IGM 8 writes that since the Rashba is only quoted in Beis Yosef and not Shulchan Aruch, the true feelings of the Mechaber are unclear. This would be equally applicable here. There is no proof he Paskens the Rashba against the Rivash, if they are arguing at all.
Finally, since when does the Shach ever hesitate to argue on Shulchan Aruch? Especially, when he has the Rivash, Rabbeinu Yerucham, Tashbetz Katan, Orchos Chaim and Kol Bo backing him up.
So, how do YOU understand the Shach?