I agree with all that you've said. I'm all for moving ahead if the rest of us are.
I haven't much else to say actually....
As for who writes first, I agree Ninjago_Builders can write the stealing of the piece, and then we can cut to introductions. Perhaps NB's chapter ends with Dewin, through his sorcerous ability, detecting that the map has been moved, and then we cut to his introduction- assuming his introduction is a chapter all about himself. But I forget how we already planned Iamos to be introduced, so however it is, he will be introduced in the next chapter written by Harold. Yes. This sounds good to me.