It is safe to insulate my uninsulated oven?
I personally would not insert any insulation anywhere inside an appliance, even if you see a spot where it looks like another model is insulated. You risk starting a fire, blocking airflow or vents, trapping moisture... Also note that the broiler will get well over 2000 °F, even a normal household candle can be 2500°F. So I wouldn't necessarily consider your mineral wool safe if it is anywhere near the burner.

Possibly you could insulate the sides or back of the oven. The manual should include any clearance requirements, if any. As long as you maintain those you could be ok.