Duncan wasn't with the pack, he helped them out every so often because he wanted to-he was Scott's friend. He wasn't an emissary... because his pack had died. He hadn't chosen to be an emissary for Scott. 

Just a friend.

It's not an issue about control, it's an issue about Pack, and Magnus is apart of the pack. 

Not to mention, it is an issue with Callies coven-Leo may be her right hand in it, but she is still the coven leader. She doesn't want Leo to go into any danger, and she wants to see the site herself, with him first, after Zoe allows them to. Why? Out of respect for Zoe, but also because she wants to see Leo's first impression as well as her own. 

From my understand of Druids and packs is that Druids become a part of the pack, simple as that,