1) You will be fine at those temps.
2) Make sure you set the regulator to about 35 psi or so. the lines are not built to with stand 150 psi and you may rupture something with to much air. I let the water out of each faucet one at a time and then redo each one until no more water is comming out. dont forget the toilet and outside water faucets. (sink and /or shower), and black tank flush. Also according to Keystones manual you should run the water pump for about 30 sec. to expell any water that is left in the pump. There is a section in the owners manual that covers what to do when winterizing , both wet and dry methods.
3) I dont know if all the water would come out of the water heater with just the air method , but its much easier to pull plug and let water drain instead of trying to blow 6 gallons of water through the lines.)