Dont worry about valve number 5, just be sure to relieve the pressure from the system. Turn off the water ahead of the softener and open water in the house. Use extreme caution when removing components from the valve, be sure the pressure is relieved. Water pressure can create flying parts!
Dont worry about the air in the brine line. If you plan on cleaning the brine tank, remove the good clean salt on top into 5 gallon buckets and when you get to the dirty salt at the bottom, I would recommend disposing of that. Salt is very regional, local availability can sometimes be very dirty. Regular brine tank cleaning may be necessary. If your local rock salt is dirty, maybe consider going to pellets. Once the brine tank is clean, refill with salt and add 5 gallons of water to it and manually regenerate the system.
The brine line is not pressurized, a little air in it will not hurt anything.