Working on the refrigerant lines requires special tools and skills that most DIYers don't have. And you'll likely need a license to get additional refrigerant. Therefore, fixing the problem will require a professional.
I'm a bit surprised that you didn't say he made any attempt to locate the leak for you. I would start searching for the leak by applying some diluted dish soap over all the joints in the coolant lines. One other place to check are the caps on the valves. If you don't see any bubbling, then turn on the AC and see if any of the joints start to bubble. The vibration and pressure changes may set off a leak. Some of these joints may be within the inside and outside units, so if you don't feel comfortable disassembling these, you can get a professional to find the leak for you. Otherwise, be very careful working around the coils, the metal fins are very thin and easily damaged.
Since it was only low, I would wait for a year and have the system serviced before the next cooling season starts. They can check the levels and let you know if the levels dropped. If they find it dropped again, they should be willing to find and fix the leak for you. Since the first person didn't seem to want to do this, you may want to get a different company to service the unit next time.
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
Is there some preventative maintenance that can help alleviate this common issue?
Other than keeping your furnace filter clean (check it once a month) and washing out the fuzz stuck inside the condenser with a garden hose every spring, unless you notice a ridiculous amount of vibration or unless you know you're low on refrigerant, then there's no reason to call a guy out and nothing more you can really do.
That "Leak Stop" stuff is a stop-gap (it's worth a shot though). If your system is low due to a leak then it's on its way out. It's up to you to spend ~$100 a year to probably continually leak Freon, or drop several thousand up front to not to.
Anecdotally, no one 'goes on' about their working AC unit. The failure rate on condensers by year five is less than 10%.
Why are HVAC systems so susceptible to failure?
It's probably the most complicated machine in your home, subjects itself to vibrations and the weather, runs at pressures upwards of 400 psi, passing refrigerant through flimsy copper or aluminum tubes that have about a thousand solder joints.
Most manufacturers have different grades of equipment, but most of the improvements they offer are for comfort or a slight increase in efficiency - but every split system ever made still has to contend with the above.
Is this the way the "industry" or "man" keeps HVAC service companies in business?
Yes, but not in the way you're thinking. The 'industry' could care less about service companies. What they care about are the installers they sell equipment to, and you'd better believe that they build them as cheaply as possible. And that's why the term builder grade A/C unit exists. It just needs to work well and long enough to sell the place. Like all things (they fall apart), it comes down to economics.
I'll take a wild guess that your system doesn't have a nameplate on it that says Carrier or Train (or any of their subsidiaries) - you get what you pay for... I'm looking at you, Goodman :\
Most Reliable Central Air Conditioning Systems – Consumer Reports
Therefore, fixing the problem will require a professional.
Well, not literally. You just need the tools, the knowledge, and a license to buy refrigerant. Free free to become an AC tech at any point... No? Well, that's why it costs so much money ;)