Tim Carter at AskTheBuilder.com has a lot of articles on French/Trench drains. He likes solid pipe and I agree. You are correct in that the holes point DOWN. Here's why - the water won't magicially find the holes if they are pointed up, but if they are down, the water can fill the trench and then flow into the pipe. Put a cleanout on the upper end if you can, if you don't want it sticking out of the ground, then come up with a 45 elbow and bury the cap right below the ground. I would NOT put a sock on the pipe. That will just clog. If dirt gets into the pipe, most will flow out because its rigid, and hopefully you can put a cleanout on it, or you can run a snake in from the outlet. I would put some straw or landscape fabric right under the top soil. Use 3/4 gravel as filler, not smaller. Read Tim's articles if you get a chance.
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
You will not have a siphoning effect unless the corrugated pipe is completely filled with water and no air is allowed to enter it, which will not happen from a gutter draining into it. No real consequences of the water sitting there. Most of the water will drain out if the pipe is perfectly level, which will be hard to do. The remaining water will be the amount that is "held in place" by the corrugated part of the piping. There will be some air flow from the gutter through the pipe so the water will probably evaporate.