[RightWhale]

what would cause it to reverse polarity under the water-vapor hypothesis?

Answer that for any hypothesis and win at least the Nobel.

Are there dipoles in the sun?

No, but due to ionization it doesn't need dipoles. The sun can generate mag fields just by churning ordinary electrical fields.

[inquest]

But at least with the core-convection hypothesis there's at least a somewhat plausible explanation, namely that the gyres change direction -or- perhaps that the strength of one gyre overcomes the strength of the other rotating in the opposite direction, and thus its field becomes the dominant field.