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
Your inside coil should have two outlets to drain condensate, one running outside and the other with a float switch to shut down the unit if the outside drains clogs. You can unscrew that float unit and put a bucket there to drain the condensate until the line thaws.
You should also figure out how to correct your condensate line so that it can not freeze. In climates where freezing of the condensate line is possible, you have to run it so that you maintain an air lock on the line (with a p-trap), but also do not expose the water creating the air lock to freezing temperatures. like this:
Notice that, starting at the a/c unit, the condensate line has in this order:
All of this needs to be laid out so that everything from the air handler through where the condensate line starts its downward slope will not freeze. Since the line is sloped down, it will drain, even in cold weather. It's also important to minimize the amount of line exposed to freezing weather in order to prevent the water from freezing in cold section of the line.
Also, in emergency heat mode, you may not be able to generate enough heat to keep the house comfortable. It used to be that the emergency heat coils had to be large enough to heat the house by themselves. Since heat pumps have been on the market for quite a while and their reliability and availability has improved, that is no longer the case.