Shut it off....just follow your normal winterizing. Run antifreeze through the pipes until all faucets (kitchen,vanity,shower,outside shower) have pink on both hot and cold. Antifreeze doesn't hurt a H/W heater. They put bypasses on tank heaters simply because they would take a lot of antifreeze. The tankless ones hardly hold any water.
antifreeze wont "push" the water out it will mix with it. I would rather have a 100 percent concentration of antifreeze than a mix. Draining then blowing out the lines accomplishes this.