OFten those shut off valves have a little cap on the side of the valve body, MAybe about index finger size around. Sometimes the cap is on the wrong side of the line.
Open all faucets and go around with a shop vac trying to suck out water in low spots. Get some RV anti-freeze and fill all traps with it. toilet, sinks. Flow extra so you can get the main trap to the septic. Sometimes some traps have drain plugs too. When going around trying to suck water out of faucets, I bring a few feet of 1/4" hose to suck the water out of the traps before pouring RV antifreeze.
Probably 2 gallons should be ok. I wouldn't be too worried If I had some of the 2nd jug left over.
That last run of line I would buy a heat trace pipe for it. You can get self temp regulating 3 to 5 foot long ones and some insulation. Hose looks like black PVC with a brass crimp instead of a garden hose.
My heat pump leaked out a line last month or so. Had to recut and flare and fill back up with 410a. That condensate drain needs to be cleaned every now and then too. But, thats summer issues, but easy to do if your walking around with the shop vac. I hose out the condensor fins twice a year or so.