You are definitely running a risk of freezing these lines in an unheated space. It is never wise to run water lines in an outside wall or above an insulated ceiling in an unheated space.
Perhaps you can do one of the following:
Run the lines under the floor in the basement (not unheated crawl space) where freezing will be much less likely.
Run the lines under the insulation in your ceiling next to the heated drywall. If you have to drill holes in ceiling joists, be sure they are at least 2 inches from ceiling to prevent accidental puncturing from drywall screws. Apply the insulation over the lines.
Thermostatically controlled heat tape can work, but be absolutely sure it is installed properly and if possible use a heat tape that you can use pipe insulation over. Not all heat tapes allow use of pipe insulation due to overheating of the PEX.
If you do have to run in this area, be sure there are no drafts that can come to bear on the tubing. Freezing is always faster if a cold draft blows on a water line.
I wish I had a foolproof method for you to use. Just be cautious, monitor the conditions and have a water shut off handy. Although PEX will handle a lot of freezing without bursting, a leak is going to be a very expensive fix and a mess to clean up. Good Luck
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
The first order of business is to find out where the frost line is in your area. The second is to dig a trench 6 to 12 inches deeper than the frost line from the immediate vicinity of an outdoor frostproof faucet (a frostproof hose bibb at the house sill is ideal) to the pasture. The house end of the trench should be pretty well squared off.
You can get ABS pipe in long rolls, and it splices very easily with nylon couplings and stainless hose clamps. Lay it in the trench. At the house end, it should have a 90-degree nylon elbow pointing straight up immediately adjacent to the faucet or hose bibb. On that end, install a female faucet adapter so it can be screwed directly onto the faucet/bibb.
You can now bury the pipe from the house all the way out to the pasture, except for the last several feet. At the end, where you'll draw water for the trough, install another 90-degree elbow and a "Yard Hydrant" (your local ag supply place will know exactly what I'm talking about). Finish burying the pipe.
At the beginning of frosty weather, you'll now build a small lean-to igloo of hay bales around the new faucet adapter. Don't try to insulate between the house and the "hayloo", let heat coming from the house wall heat it. A typical hay bale's R-value is about R-50, which should be plenty to keep the hose connection from freezing over the winter, considering you're all the way down in balmy NJ. If there's ever any question - say an extemely deep freeze - you can add a little heat with an extension cord and a CFL light bulb (an incandescent may be hot enough to set the hay afire).
Over the first winter, put an outdoor thermometer inside the "hayloo" and another outside it. Every time you remember to, go check both temperatures and record what you find. Then you'll have a very good idea how a hayloo performs, so you can predict about how cold it'd have to be before you add electric heat.