Bill,
You got all my questions, but of course I now have a new question.
One of the benefits of some of the bubblers I've seen is that they don't let the water flow back up into the tubing. If I leave the end open won't the water fill the tubing at least up to the water level? If the water fills the tubing it will be more likely to freeze and stop the air flow possibly burning out my pump.
I'll do my best to record the temperatures at which the pond freezes over and the size of the opening this winter using the open end. Then next year I'll try and compare that to when I have a bubbler. I still think someone should donate some money towards this project in the interests of science so I could get a second pump and test them in identical situations.
I'm pretty sure the plug is a ground-fault plug, but I'll make sure. Isn't it required in outdoor plugs on newer homes? Our house is only a few years old. I am also planning on using the plug that is connected to a light switch inside so whenever I am dealing with the pump/plug I will turn off the switch so there won't be any current. I'm not letting the wife collect the insurance policy THAT easily.
