Adding sauce without microwaving the chicken

That won't make it non-safe as long as it gets hot all the way through.

MICROWAVE (PREFERRED METHOD)

• Place bag of sauce in a bowl with warm water to thaw.
• Open bags of chicken and broccoli and empty into a microwave safe container. Microwave on HIGH, uncovered for 3 minutes.
• Open bag of sauce. Stir in sauce and heat for 3 additional minutes or until hot*.

NOTE: MICROWAVE OPERATING AT 1200 WATTS. MICROWAVES MAY VARY, ADJUST COOK TIMES TO INDIVIDUAL MICROWAVE.

*Internal temperature should be 165°F for at least 15 seconds. These instructions are guidelines only since individual cooking equipment vary in temperature.

From the manufacturer

It's the 165F (74C) for 15 seconds that is actually relevant for food safety. That's just quite hot all the way through, and is almost certainly an overly conservative recomendation anyway.

They specify not heating the sauce as much as the chicken because it's not necessary to heat the sauce for safety. The sauce can burn more easily than the other ingredients, which of course would affect taste, but it's not a safety concern.

Cooking the sauce in the microwave is not going to make it unsafe to eat. It may make it a little thicker or thinner but not unsafe.

It should be safe as long as the chicken is cooked, but they discourage it because the sauce cooks at a different rate from the entree. Dark, thick sauces like oyser sauce burn extremely easily in microwaves.