Mold is created with a combination of factors, moisture, temperature and air flow. Do you run your exhaust fans when lights are out? Does new air vent into your grow space or do you have it circulating? What's the temp and humidity of your grow space when lights are out? You said yourself that you have a lack of airflow to the bottom branches. A $10 fan may solve your prob.

Trimming just the tips of your fan leaves that are touching the soil will not stress or stunt your plant. Only when you trim too much of the plant at one time will the plant growth be effected.