Username Trails on the forums here has his own website TrailMeister - The Largest Horse Trail and Camping Directory in North America
and has played around a lot with packing lightly. That sounds more along your line of fun as poor college students as it only requires one horse per rider instead of pack animals. I would highly recommend spending some time investigating this site and reading the articles as they are very informative.
Now to answer some of your questions from my experience:
-You can pack lightly and use just one horse, how much you need depends on how many nights you intend to be gone. If you can't let your horse graze in the area you wont be doing much packing with one horse.
-Grazing horses. You can hand graze (yawn), hobble or stake them out. For hobbles or stakes you had best practice with your horse prior to going camping. Trying to work out the kinks on the fly doesn't usually work well.
-1 gallon zip lock bag holds about 3 pounds of sweet feed, half a bag in the morning and the rest in the evening works quite well along with grazing.
-State/federal parks where I live require processed feed or certified weed free hay. Get caught with feed that doesn't fit those two categories and you are facing quite a large fine.
-Sleeping. Share a two person tent to cut down on the number of tents. Use a hammock (dry/warm). Use those cheap plastic tarps and make a cover to sleep under. Buy light weight tents, sleeping bags that pack down to almost nothing. Easy to find at most outdoor stores. They also have sleeping pads that get real small for packing that you'll appreciate having if you've ever slept on a rock or two.
-High line or portable pen both work. Again it's about training your horse to them before going. My personal preference it high line.
-Don't worry about bears attacking your horse, not likely to happen. Do worry about them raiding your larder....That's why you hang food up in the air out of reach.
-Shoes or barefoot. Depends on the condition of their feet and the terrain you'll be covering. Poor hoof walls, shoe. Lots of rock, shoe. Bad feet in general, shoe. Alternately you can go barefoot. In either case take boots along just in case your bare foot horse gets sore or your shod horse loses a shoe.
-Taking care of yourself....How much can you stand your own smell?!?! Take a tooth brush for sure. Baths are haphazard, depends on the time of the year and how much water is available. Take a bar of soap and a chammy towel so when you have a chance you can. Chammy towels are nice because they dry quickly.
-Saddle to use is quite personal, what fits your butt the best for long rides? For packing you do want latigos and D rings to tie to/with.
-Food. I take freeze dried food for breakfast/dinner while lunch is just some kind of snack that will stick with you like peanut butter crackers. Take a portable stove to heat water in so you can use hot water for your meal and to make coffee or tea. Cold water works too but if it's cold outside you'll want the hot food. Again check outdoor stores for a light portable stove. MRE is the military acronym for the freeze dried food they provide troops while in the field. One piece of advice for cold weather packing, buy meals for two instead of one, you'll appreciate the extra calories in each meal.
-Water purification. Tabs or filter pumps both work. I prefer filter pumps myself as I prefer not to have to strain the junk/dirt out between my teeth. Using both is the best and safest way to go. Yes you can boil your water too but the last thing you want to do is run out of water mid day, stop at a creek, unpack your portable stove and boil the water then let it cool all before getting that drink of water you crave...Don't worry about your horse and drinking that stream water, they'll be just fine.