I've already run the gamut many moons ago of the gear path you are on. I'll address your points.
The mountain men converted to percussion and the whole frontier converted to percussion because the beaver trade didn't last that long, there was a lot of fighting, and percussion caps are simply better in every way. The glory days of the beaver fur trade, the fad which drove a frontier culture and fueled it with money and trade goods, really ended in the 1830's, relatively soon into the percussion era, and the mountain men became guides, typically for the Army or for settler wagon trains. Both of those often meant fighting. Furthermore, the advent of the revolver, which first appeared in 1836 combined with all of the fighting, helped to really accelerate the move towards percussion caps.
Keep in mind that America typically fights a major war every 20 or 30 years, with usually a lot of smaller fights in between. Wars drive technology, and the military tends to heavily influence what guns American civilians use. Much of the use of guns on the frontier was for fighting, especially on the early frontier, which included the fighting with the Indians and the Mexican War. Then came the Civil War, followed by more wars with the Indians, especially the large plains tribes. All of these wars were fought over a vast area of land in all sorts of weather.
For most things, including fighting, percussion caps are far, far superior over a flintlock. Kit Carson's lopsided victory during the later fur trade era in a wild fight with a Comanche force that outnumbered him almost 20:1 showed everyone just how effective repeating arms were, and repeating arms of the day relied upon percussion caps. After the Civil War, percussion caps were on their way out and many percussion revolvers were converted to fire cartridges.
Yes, you can use other types of rock in a flintlock, but really it's not that great. But yes, you might have some non-flint in your area that might get you by. Just keep in mind that every time you fire you add a bit more wear to the frizzen. Or, you can simply do what most people did on the frontier during most of the 19th Century and carry a supply of percussion caps.
When it comes to accuracy, the old flintlock smoothbore trade muskets really sucked. It was said that a man hit at over 60 yards with a ball fired from one was truly unfortunate. The Indians simply adapted the musket to their hunting tactics. Out in the west and on the plains a typical Indian musket had the barrel chopped to about 14 inches by the gunsmith at the trading fort. They would kill a buffalo by riding up to one like a mad man and firing into it point blank. They never really got a grasp on long range accuracy and any rifle they got soon turned into a smoothbore due to their common practice of using river gravel as birdshot.
The main benefit of a flintlock now, aside from historical hobbyists, is to have something which goes bang that gets outside of legal restrictions. Unless it's a rifle you really don't get any real range benefits over a modern crossbow, and in fact a modern crossbow has a higher rate of fire, less noise, and it's more weatherproof. It's better under low light conditions and it's typically far more accurate than a smoothbore flintlock.
All US states except Oregon now allow various types of hunting with crossbows. As a result, crossbow technology here is rapidly getting more impressive with every passing year. Furthermore, in just about every US state, American law dictates that for hunting the minimum legal draw weight on a crossbow is 120 pounds (for a regular bow it's 40 pounds). This means that virtually every production crossbow made here is at least that strong. That means out of almost any American made hunting crossbow, a crossbow bolt with a broadhead will typically go completely through a deer, and that is just a recurve, not a compound. Some of the compound crossbows push a bolt that is more of an arrow downrange at over 400 feet per second. Most crossbows now are sold with a scope and they are accurate enough to take advantage of it.
So, why use a smoothbore flintlock other than for legal reasons? If crossbows are illegal where you are (what in Australia isn't illegal nowadays), just get a regular bow.
In the USA flintlocks are mainly a niche item with collectors and historical recreationists. They really are not that practical for hunting, and in wet weather they truly suck. I've also fired them at night before and all I saw were spots for a while. Only one state requires their use during muzzleloader season.
Hunters with modern muzzleloaders probably outnumber historical recreationists 1000:1. Furthermore, unlike a modern cartridge firearm a modern muzzleloader or a crossbow doesn't require any paperwork or a background check in most US states. No one knows you have it and it's not that uncommon to see one used during regular modern firearms season due to expense, one gun for two seasons. For example, go into any almost Walmart in the US southeast and you will find modern inline muzzleloaders for sale.
Even amongst the recreationists, at buckskinning events like a modern day mountain man rendezvous most of the guns are caplocks.
For example, I can take you to any sporting goods store in Texas and get you a bow, a muzzleloader, a modern firearm, or a crossbow. However, if you wanted to get a flintlock, you will have to seriously look, or order one online.
As for parts, a flintlock actually has more parts than a caplock, and more parts means more that can break. There is a reason gunsmiths were in demand at every trading post. There is also a reason most mountain men carried more than one gun. In fact, the gun most often carried across their saddle horn while riding was actually a double barrel shotgun. The rifles rode in scabbards. Most usually also had a pistol or two.
Ask yourself, just how much is a can of percussion caps and how compactly can they be stored vs how much you actually shoot a flintlock? Even caplocks were not fired that much back in the day. When the lever action Henry rifle came out they called it the 'gun you loaded on Sunday and fired all week'. Once you get to that point it's a natural step to just transition to 209 primers which is what most new muzzleloaders in the USA currently use now. You can even get kits to retrofit older caplocks.
So, once you leave the need for being historically accurate and if you are not hemmed in by insane legal restrictions, for long term sustainability muzzleloaders of any kind really start to pale vs a break action or pump action shotgun that uses shotgun shells which you can reload. It's basically all of the same ammo components in a much more convenient, ready to load package. Furthermore a modern shotgun you can add a rifled barrel or a rifled choke and get some real accuracy out of slugs and pumpkin balls.
Really, for long term sustainability no matter what, aside from a shotgun and a small bore air rifle, get a good, professionally made bow and learn to use it well. Learn to make arrows.
Check out the folding bow these guys sell. I have one and it is extremely useful and handy.
http://www.goprimalnow.com
