As well when the ice forms it needs the pressure to stay in the pipe.

That's the reason I asked. So, you need to prevent any air from getting out, correct? or does it matter?

You just need to plug so that the water doesn't leak out. The stinger and pressure nipple area will freeze first as will the inlet where the header connects. Once those areas freeze the water won't leak anymore. Remember, that water expands as its turning into ice, so having an air tight seal isn't required.

yup, as simple as that

fill it only about an inch from the top, if its not enough, add a little more. If you put too much in it could split.

Yup!

Too much of a good thing isn't good. I have hear of people using milk because it doesn't expand as much as pure water.