Okay, this is a clunky workaround, but if you buy TWO staves (can by any combination of implement and weapon version) and equip them BOTH, you get your +1AC from defensive. I am not seeing the damage die issue though, so I don't know what to suggest there.
Except maybe this; if you want better defence and a higher attack bonus, you can spend your feat on a Singing Stick (from Darksun - it is selectable). It's a +3 proficiency mace-class (so gets upp'ed from D6 to D10 damage with Summer Druid) defensive, offhand weapon. That way you can still equip a light shield for even more AC. It's only D10 instead of D12, but it's probably worth it for the same feat cost.
Edit: Oops. My bad. You'd actually have to dual-wield for that to work. Still might be worth considering.