Togo: argh, the thought of a weekend without Internet is horrifying
I don't know what I'd do... good to have you back
I edited my last post for the Knowledge check and Benign Transposition flavor text: yes, you can assume Umbriel is attempting it. If it works, Umbriel's Phantom Steed doesn't get transposed, so it's either standing aimlessly on the street (under where Derk transposed in), or returning to its caster, or doing whatever else Togo feels suitable
If the BT doesn't work for some reason, Umbriel will go back to the corner on her steed, shoot Derk an apologetic look, and watch the show.