Just start your horses with you around to keep an eye on them. Keep the Highline higher than their heads, so they easily walk underneth it. Tie their leads short so they can't get a leg over the lead if the try and scratch their chin. The old standard rule is 7' high, 7' aprart and 17" of lead.
I do on occassions tie the leads a little longer so they can get their nose down to the ground to eat. I usually do this when I am around to watch, If left unattended, I tie themback up short.
Any water buckets I left around highlines usually get kicked over and horses get their feet tangled in the bucket, So I bring water and offer and then remove the bucket after they have drank.
I do get tired of wasted hay or pellets. So I place most of the feed in plastic Tubs and place it under the nose. When they are done eating I can drag the tub out of reach.
We've done lots of camping over the years, Early on, we would pack one horse and hike in leading that horse. As we got more horses, bigger trailers, I started packing one horse for 2-3 people. I'm a big enough guy that I don't want to add extra weight to my saddle mount. I just never carry more than what I can fit into my saddle bags and coat tied behind the cantle. Although I've seen friends put their lighter kids on horses and load pack gear on with them.
I often hunt off my horses. When I shoot an elk, we carry panniers behind the cantle and get off load the panniers and lead the horse back to camp.
My daughters have enjoyed many a camp out with their dad, leading an extra horse across the mountains
At any rate if you're going to be camping off your riding stock the main considerations are preparation, weight and volume. Preparation is key because you wont be able to carry the amount of feed that your horse will require and you'll need to camp in areas that have grazing (make sure your horse is hobble trained)
Trying to stay below the magic 20% of horse's body weight tough real tough. It can be done but you've got to be ruthless in eliminating what you don't need. For example your knot eliminators weigh a good deal. Learn a few good knots and use those instead. (I use a prussik to attach equines to a highline)
Similarly, volume or the space that your gear takes can easily become a trap that keeps you from dismounting in an emergency.
Here's a link to simple pack weight calculator and pics of the gear I currently take for an overnight trip. Packing Weight Calculator
And here's a link to an older page that shows what I used when I first started camping off of my saddle stock. The weight was marginal but the volume scared heck out of me. Horse Camping Without Pack Stock
You should also check out the upcoming April, 2012 issue of Trail Blazer Magazine. The feature article is 10 pages on exactly your questions.
I really enjoy horse camping without pack stock and hope this helps.
I started by having a friend walk off 10 -15 feet away and fire a smaller caliber pistol away from the horses while I held their lead and calmed the horse. After a couple of shots the horses usually stop startling at the noise and start to accept it. You can then move closer and closer where you can shoot the pistol yourself.
I try to never shoot in the direction of my horses head. ( not talking about bullets here, but where the muzzle blast and noise are directed) I don't want the noise to be directed at their ears. After 60 years of loud music, shooting guns, being around military aircraft and construction sites, my hearing has suffered significant hearing loss. I need my horses to listen for boogymen when I'm out in the wild. Because I can't hear them. I don't want them to have any noise induced hearing loss.
Folks involved in the Mounted Shooting groups that shoot from the saddle in competion, often put ear plugs into their horses ears to protect their hearing.
What you want your horse to learn is that a Gunshot is not a reason to spook. Or least the surprise of a gunshoot should only startle a horse, but they should spook in place. My horses even after years of hunting will still flinch when a gun goes off. But they don't get scared and try to run off.
As a hunter I never fire a weapon off my horses back. There is no way I could be accurate. I always dismount, and walk a few steps in front of my horse before taking aim and firing a gun. 1st of all this puts my horses BEHIND the noise. They are watching what I do, so its not as surprising to them. My horses have learned to stay ground tied even when a rifle is fired. But if I had to discharge a large caliber pistol to scare a bear off, I'm not worried about firing from the back of the horse.
Your other question, I've never found a horse that would not highline. They all learn this very easily. Maybe they learn this because after a 20 mile ride they are comfortable being highlined with their herd buddies with hay below their nose. Why would they not be content to stand in that place.