I don't get it why he is giving you such a hard time when you a paying customer, not to mention there is a simple fix and it can be done in 5 min without disturbing existing molding. All he needs is 3 short pieces of molding, pre cut them, nail them together and put them in, simple as that. Maybe he thinks that existing molding also needs to be taking down in order to this, but it don't.
Here is a detail what needs to be done (I'm sure he knows this), show this to him and hopefully he can do this for you,even if it cost you a little more, it will look so much better.
Good luck.
Q
Crown around the Vent
Q