I'll try to answer your various questions so this will be a little choppy.
Opinions vary a little bit but most most concur that by age 3-4, they are ready for full loads. You can pack them younger but they shouldn't carry as much, maybe 10-15% of body weight and not strained by the terrain. You can expect them to pack until around 12 years old and some longer.
As for leaving them unattended in camp, that would make me a little nervous. A lot of guys do, especially if someone is around to keep an eye on them. The concern about leaving them is predators, ie bears, mountain lions, and even dogs. Having them in an enclosed pen would would be a lot better. In camp, goats are curious and mischievious. They'll knock stuff over and get into things. Some are better than others but you will likely want them penned or tied up. I have kept them in a horse trailer with food and water which worked pretty well. They had a enough room to get up and walk around with food and water readily available.
As long as a goat is not left alone, they seem to do just fine without any seperation anxiety. Leave one alone and he thinks he is going to die. They are herd animals so at least keep them in pairs.
I don't think that goats are going to engage in any blood tracking, they didn't even seem to notice all the blood. They will however walk all over existing tracks and blood making it more difficult to follow a trail.
I got my crossbuck saddles and panniers from Northwestpackgoats.com. Great products from a great company. The mountain straps option is very important in mountain terrain.
Goats really don't cost that much. Kid's, in our area, cost between $40 - $125. For my last kids I paid $125 and they bottle fed them for 3-4 months until weaned. Kids are really fun at those early ages but they require extra time and money for the bottle feeding. Experienced adults can run as much as $600. Fencing can be one of the most expensive costs. The can jump over high fences and get through many others flawed fences. To solve those concerns, I ran a Petsafe underground dog containment system (I actually just hung the wire around on an existing barb wire fence). The goats wear electric collars that remind them that the should stay completely away from the fence. It's has worked very well for me. The other ongoing cost is feed. I only have to supplement their feed in the winter and have done it for less than 2 tons of hay and we have long winters. If you had to feed all year, I read that you should plan on a ton of hay per goat per year. There are ongoing costs but nothing like large pack animals. It really doesn't cost much, but it adds up when you have seven goats like I do.
NEVER, NEVER, NEVER buy a goat that is people shy. If they don't come right to you and want to hang around, don't waste your time. I am speaking from experience on this one. DON'T GET THEM. Why? They are not likely imprinted on humans and if spooked may go running off the mountain with all your gear (again speaking from experience). Imprinted ones will come running to you and are so much easier to deal with because you never have to try and catch them because they want to be with you.