I have used bag/ horn combos set up both ways for different guns - some attached together, some on separate straps. Overall, I prefer having a medium sized horn, attached to the bag straps. Just my preference, and I would suspect that in the 1700s it probably was a matter of preference as well, especially once you were into the period of longrifles and hunting pouches as we think of them today, i.e. latter part of the 18th century.
I find that horns on separate straps tend to want to work their way around out of place when moving through the woods, leaning, stooping over, etc. and tend to swing out in front of me and hit on things. And if you are carrying a canteen, haversack etc. a separate horn strap is one more thing to arrange and wear around your neck that is unnecessary. I have never found a horn attached to the pouch to be cumbersome or get in the way when loading, if you put a little thought into where to attach the horn straps and how long to make them. I could see where a very large horn, 15 inches or bigger, might tend to get in the way a bit more, so perhaps there is one reason for carrying it on a separate strap.
This is just my opinion, but I think there is overgeneralization about the way things were done in the 1700s. I know Madison Grant's book states that most 18th century horns appear to have been carried on separate straps, but I am unclear how that conclusion was reached since to my knowledge, there are no documented 18th century pouches with the original strap intact, are there? Not saying it is not true, I am just not sure there is much evidence to base it on. And as Mike pointed out, while there are some good early European depcitions of their colonists and officers, there are frustratingly scant few, if any, "in the field" portraits that clearly depict late 1700s American frontiersmen, settlers, etc, equipped with longrifles and hunting pouches with enough detail to glean much about how they did things. Studio portraits or portraits done many years later, like the Chappel portrait of Dan Morgan, or the Boone/Harding/Lewis portrait - done from a head and shoulders sketch made while Boone was propped up in bed, with the body added later and finished in a St. Louis studio. - can be misleading. I would expect a settler moving from an area with a well established rifle culture, like Virginia or North Carolina, through Cumberland Gap in the 1780s or 90s might be equipped a bit differently than a provincial officer like Robert Rogers or Guy Johnson in a studio setting done 15 or 20 years earlier.
As far as how high they were carried, again, I think preference would prevail. A few years back a lot of guys started slinging their pouches really high, perhaps because of 19th century photos, and it does tend to keep the pouch and horn securely in place while moving around in the woods, especially if you have to move fast. But as some others on here pointed out I find it really awkward and uncomfortable to do so as I have to bend my hand backwards to reach down in the pouch. If it is a bit lower, I can reach straght down into the pouch. There are a few 19th century paintings that show pouches slung in the waist area - between the hip bone and the lower rib - whether this is how they did things in the field I don't know but I find that to be a good compromise between keeping things securely in place, and allowing good access. What works for an individual will depend on their body shape, size of the horn, etc. Maybe even length of the gun barrel - I find it preferable to keep the muzzle as close to straight up as possible when loading, but with a 47 or 50 inch barrel, it is pretty tough to do that with a measure attached to your strap unless you are really tall...or have the accoutrement straps for your measure etc. hanging down really far when not in use.
Fun to ponder
Guy