I wouldn't worry damage from a bronchodilator, as long as you do it as directed. You are actually giving your lungs a break by relaxing the airways and making it less of a struggle to breathe.
If you have an inhaler, you could use it before the saline, instead of 2 nebs. Opening the airway will bring up even more mucus.
Keep up the clearance - it will help keep you from getting the next lung infection.
Sue