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.
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.
Best Answer
Heat tape is not the first thing I'd suggest (I have considerable experience with the stuff, and that makes me avoid it whenever possible.) PVC and heat tape can be a little bit tricky as PVC does not take heat well and also does not conduct heat well, and heat tape is a problem in the long term as it tends to fail and can suck a surprising amount of electricity over its life.
If the pipe is just freezing at the exit (open drain) where it's more shallowly buried, I'd start with digging up about 4-8 feet at the end of the pipe and placing 1" or 2" XPS (Extruded, usually blue or pink styrofoam - the waterproof stuff rated for underground use, not the expanded white stuff that crumbles) over the pipe and reburying. By insulating sideways 2 feet either side of the pipe you gain the effect of burying the pipe 2 feet deeper, so it's much more resistant to freezing. Just expose the top of the pipe, and a 4 foot wide strip centered on the pipe, place insulation 4 feet wide, and replace dirt and sod over the insulation.
You could also, with not much more work, replace that final section of pipe with a 4" pipe (in addition to, not instead of, insulating it.)
An alternate approach that's more work would be to change the termination of the pipe to a dry well, where the end of the pipe is not out in the weather at all, and just dumps into a buried, perforated barrel filled with large-size drain rock. If the burial depth is still shallow, you might still want the styrofoam on the last bit of pipe.
If you are fixed on an electrical solution, "inside the pipe" heat tape is generally more reliable, but would be dubious for this application since the pipe is not normally full of water.