Clicker training works very well. Add treats and you're in a good training place!
I think that the hide and seek will be fun. Missy's nose is going to lead her to the items. You may have to up the difficulty level if that happens.
And, one word of warning: While you don't want to make it too easy, so will want things under and behind items most likely, they can become so into the game that they just flip over everything in their way. So, anything that you don't want knocked over should not hide treats! But, piles of stuffed animals, under pillows, etc. work fine. Although you'll probably have to pick a few things up later on! Oh, and don't use the couch as a hiding place...dogs can basically dig to China in one if there's something that they want there! Don't ask me how I know that.
About Kohle...perhaps suggest a different outfit and accessories. And, when she's around Chelsea, watch to see if you can detect any behavior that would cause a reaction. Right now it's reversible. You don't want it to become a normal reaction to her.
It can be anything from stepping back from her, to picking up her toy but not playing with her with it.
It must be something, because the girls don't dislike Kohle, I'm positive.
Also, I'm not past bribing a dog to elicit the response that I want. It may be that Kohle should hand out a few high end treats when she's there. No one else, just Kohle.
Love All Life, Thank You For Posting! :)