LOL! I didn't use mustard, probably because I didn't even read the instructions! Hmm... maybe I will go back and add it.

I would probably use dry mustard. Not sure how much. Maybe 1 tsp. One of the commenters used 2T. of Dijon, that seems like a lot.

This recipe uses 1 tsp of dry.
http://allrecipes.com/recipe/poppyseed-dressing/

I'm fairly certain the Costco version didn't have mustard. It was very white.
I didn't dissolve anything into anything. I just poured the dry ingredients in the blender and blended it well, then I added the oil and xanthan gum.

Yeah, that was what I meant. "Just Like Sugar". You could use almost any sweetener you have on hand, except I would be wary of stevia. It might work, but it can be hard to work with.

Here's that Sweet Kale mix from Costco. Awesome ingredients and VERY good with the sweet dressing. I'm not sure I would like it without the off-setting sweet flavors because those greens can be kind of bitter.