it makes your weapon a magic [blunt] weapon that rolls Wisdom to hit instead of Str or Dex... in the early builds is was consentration which broke every combat so Larian changed it to 10 turns the same as invis [many other effects]
now it also follows other combat rules so you get extra damage for 2handed and yes you can carry in the off hand... personally i think Larian just fucked up their text when they made the change as the point of the spell is to get Wisdom to hit not Wisdom to damage but a build that doesn't have either Str or Dex is very uncommon anyway
I do not know if I undertood correctly. But point of Shillelagh is to get spellcasting modifier to hit and damage both.
