This is a bit of a strange spell. It's conjuration (summoning) spell, but works like an evocation spell. Soo...
1. Can the Dazing metamagic spell be added? Is it the spell doing the damage, or the summoned ancestor?
2. Can the summoned ancestor spirits provide a flanking bonus?
My guess is;
RAW dazing works because spiritual weapon strikes as a spell and is subject to SR. Can't flank because spiritual weapon can't flank.
RAI dazing doesn't work. It's the ancestor guardian that is dealing the damage, and the entire reference to spiritual weapon is just there to govern how attack and damage rolls work. Sure, they can flank.
What do you all think?