Mustard powder absolutely acts as an emulsifier here. The mucilage is a polysaccharide which helps suspend the oil in low egg systems like miracle whip, but admittedly the cornstarch is doing more here

There is no direct msg in miracle whip, nor does it contain things like yeast extract or hydrolyzed proteins for natural glutamates to avoid labeling. At least in the USA these things cannot fall under the natural flavors loophole and msg must be labeled

It is an emulsion of eggs and oil (eg a mayonnaise) that is thinned with water and then thickened and flavored