Any or all of your ideas will help, it largely depends on your climate and how long of a below freezing stretch you can expect. The drain outlet is supposed to be within some distance of the ground, 18" I believe, which is why it was configured that way. In areas subject to freezing, all piping carrying water should be run as far as practical inside the heated envelope, so the drop to within some distance of ground should have been done inside. If possible, rerouting the run inside, with only a very short run outside should solve the problem.
As BMitch suggests, routing the drain through trap would comply with plumbing codes and is the best solution.
Shortening the pipe could fix the problem entirely since the water may not have time to freeze. In frigid climates though, any water outside will freeze. The high outlet could damage you outside wall finish and be an annoyance to passers by.
Widening the pipe will require more build up before blockage, so if the freeze periods are not too long, this could work. In frigid climates, it will still block up eventually.
Insulating the pipe would probably be all that's needed, it has the same effect as shortening the pipe, without wall damage. Actively heating the pipe would certainly work, but is probably only needed in extreme cases. Heat tape that wraps around the pipe and is plugged in to an outlet, with a thermostat to turn off the current above freezing is a common hardware store item in cold climates.
You only have to worry about the attic if it gets below freezing in there. Attics, though ventilated with outside air, tend to stay a fair bit warmer than the outside. I would guess that unless you have good runs of outside temperatures consistently below 0F(-18C), you shouldn't have a freezing problem in your attic.
I'm not sure, but I suspect there is not an override. A blocked drain can cause significant problems if the furnace continues to run. By shutting off the heat, it is good incentive for the owner to solve the problem.
Best Answer
I would strongly suggest an "in the pipe" electric heating solution. Not to run all the time, but to be there if you need it, since this can be imperfect, to say the least, and it's a lot easier to plan ahead (and just be able to flip a switch until the water runs again) than to fix it when it's already frozen. I would not recommend this "instead" of the insulation technique below, but in addition to, as a backup. It just needs to run long enough to permit some water to flow, then you can shut it off and run water until the pipe is fully cleared of ice.
The non-electric approach (other than blowing a channel into the bedrock) is to excavate a WIDE trench to the bedrock, and place rigid foam insulation over the pipe. If it's down 2 feet and insulated sideways 4 feet, it's morally equivalent to being down 6 feet. That means an 8 foot wide trench and insulation, to get 4 feet to either side. You could also mound additional soil on top of the pipe run.
Running a "controlled leak" through the winter is an additional technique - typical well-water is 50F/10C, so running a little water all the time helps to keep the pipe from freezing.
You might also look up the well driller or plumber who ran the water from your well to the house and ask them what they did, if it's working.