Mac and Cheese advice

The mustard's primary function in this recipe is as an emulsifier so the sauce doesn't separate.

The cornstarch's primary function here is as a thickener.

I have never used mustard in mine. Must be for flavor omitting it glis fine. Season for 'taste' with salt and oepper

You don’t have to use mustard, not required element. If it’s too thick you can add some milk . Don’t worry

I've always found the mustard in Mac & cheese more a seasoning than a thickener. You could make the recipe w/o it and it would still be great. Optional replacements: prepared mustard, wasabi, horseradish sauce.
As for the cornstach, maybe someone else can weigh in on what it does here...maybe smooths out the sauce. If it were me, I'd omit it.
Hope it turns out beautifully.

Use 1/4 of a tablespoon of regular mustard (eyeball it), and then omit a little (about a tablespoon) of milk from the recipe (to make up for the liquid). But omitting the milk is not necessary, as it's stovetop mac and thus meant to be a bit saucy anyway. Good luck!