What should you do if the low pressure line is insulated and sweating outside the insulation above a suspended ceiling and dripping on the ceiling causing mildew?

What is your question here? If you want to keep the insulation from sweating get thicker insulation. You probably have 3/8 thick insulation; increase to 1/2 or 5/8. It is possible that your suction line is freezing up and when it thaws it soaks the insulation causing it to drip. If you can't prevent the leak, use a vinyl ceiling tile, like Ceilume, in that area. It won't absorb moisture and is easily cleaned.