For part one, there are two aspects to consider: one is the regular old hit to the cardiovascular system that makes you tired more quickly because the air is thinner. This will affect you and your dog pretty much equally. However, many dogs will run until they are totally exhausted whenever there is adventure involved, so this aspect at worst it will just slow your dog down and he'll walk with you instead of excitedly running around.
Two, there is also the issue of altitude sickness, which is a little bit different, with potentially serious symptoms. Like humans, dogs and other animals can get altitude sickness. However, they aren't able to speak and tell you that they are feeling nausea or has a headache. This aspect is important because many of the symptoms are similar to just getting tired, which unless you take steps to prevent, your dog will happily run itself to exhaustion, which will also aggravate the symptoms.
With that said, altitude sickness is not really a concern until 8000 feet, and if your dog is in real good athletic shape, he should be fine to 12,000 ft, but keep a close eye on him. I think too if you do your hike with your dog on leash, everything should be ok.
As for part two, from most dog packs I've seen, and from the packs I have for my dogs, you can pretty much jam them completely full of food and or water, and they'll hardly bother your dog. After all, your dog has more important things to sniff and explore than to worry about a little bit of junk he's carrying. I think my dogs could easily carry a weeks worth of food in terms of mass, and have plenty of hauling capacity left, though that is about the limit in volume of the packs I have. You will however need to supply them with plenty of water, which is also important in staving off altitude sickness.
