Sorry for the silence - I had a weekend without internet !
Given that there is nothing in Umbriel's knowlege check that suggests resistance to teleporting attacks, I'll assume that's going ahead.
The mirror is the cart is a one of the large 'glass-steel' mirrors and is intended as a decoy. Nystal's magic aura
has been cast on it to make it appear magical. It's worth about 3-4000gp. It would be a shame if you lost it, but it's not magical and Kalinor probably wouldn't be worried if it went missing. He has more.