first, constantinople didn't close. they are still active and sodom is trading metal and many other things, so are other people.
as far as diminishing returns on varying skills, the truth is that all successful villages already have specialists. a player with 200 survival, a player with 200 carpentry, a couple folks with 200 farming, and a player with 200 smithy (quick example) are much better off than 5 players with 40 in each skill. progress and quality are made by people collaborating like this.
jack of all trades, hermit types are already handicapped enough as it is however. they lack the protection of friends, and because of the example listed above, their products will suffer greatly in quality already. however, hermit should be a viable game choice because some people don't play for the social aspect or the quality race and there's no need to make it worse for them.
as far as mine ownership and crop scarcity/animal decay:
remove crops/livestock from mine areas, and people will either just do without, or just make satellite villages over their mines and live in farmland, and collaborate among themselves. creating an artificial scarcity of food won't encourage trade, it will just be an annoying mechanic that people will find their way around.
having said all that, there is no "everyone has everything so why trade problem". some products like linen or cavebulbs will always be in demand. or if you can make decent quality pastry, sausages, or cheese you should have no trouble trading them for metal and other things you want. remember there is an opportunity cost with producing metal, mining and smelting are time consuming, as well as use up a lot of food. and what this means is less time for those players to farm and thus a demand is created.
to sum up:
get to constantinople, i'll trade metal for cheese or linen or other farm products and so will many other people. if you are far from nople or can't find it, explore your area and find a mining village to trade with. hell, you might even find some forgotten old mine that someone's left behind, and do a takeover if you are motivated enough.
most mine owners understand they have an unsaid responsibility to trade metal for those without, and should be able to work out a deal with you. what that means though is that you'll have to not be lazy, and produce goods, the same as they are.
if you are able to impress someone with your production as a farmer, hunter or resource gatherer, you might find your way into joining a successful mine village if that's your goal.
finding a mine and using it to produce metal requires effort. so should making the goods that you would trade for such metal.