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.
With my furnace, I diverted the drain line into a 5 gallon bucket which I used to flush toilets. You could install a wye and a valve or two to make the condensate drain into a bucket inside the house on the coldest days. This may be a little too high efficiency, Worked for me. The tricky part is remembering to check the bucket.
Best Answer
I would start with boiling water - a couple of very large pots. Dump. If you get any kickback or notice that your water levels stay high repeat a few times. If you have ice or a blocked line, really hot water helps both.
Normally you would check your water levels in a cleanout or basement drain - from the sounds of it you have a bathtub at your low point or maybe it is at the same level. Usually I tell people to have one person look with flashlight on drain. If you see the water levels rise, even a little temporarily then usually there is some sort of blockage - I know that it may be ice in your case.
If the water isn't backing up into tub move on...
Then I would fill showers/sinks/tubs - anything that holds hot/scalding water and fill them until you run out - except for said tub that got the backwash. Again good to have another person at tub/drain. Dump everything at once. If it starts backwashing to tub then stop. You have to think just after a few times doing this that ice would be thoroughly melted and then you need to troubleshoot like your main line is clogged.