Well, to make a desert planet habitable, you will need oxygen, unless your 1870's cowboys live in domed towns, which seems incongruous. Humans need to breath oxygen! You will also need water (could be subsurface) and crops: Which means whatever factor killed off all the plants (including the forests turned coal) must no longer exist, or you couldn't grow anything new.
If you want the biological cause of the non-conductive metals being present, they will need water, too. Bacteria aren't a dry collection of sticks; they need water as their primary solvent.
Some of those problems I cannot solve in this post; perhaps I will have an idea later. The oxygen problem can be solved; however.
Until recently, it was believed oxygen would have to be a sign of life on an exo-planet; for example see this link on Signs of Life. An excerpt follows:
What would be a real biosignature? Many scientists think that abundant molecular oxygen, or its product, ozone, is a strong one because on Earth molecular oxygen is produced mostly by the photosynthesis of plants. The simultaneous detection of water, carbon dioxide, and oxygen or ozone would be a strong indicator that biological processes are occurring on a planet.
Also, the detection of large amounts of oxygen and a gas like methane or nitrous oxide would be a strong signature, because on Earth these gases are produced almost entirely by biological processes.
Also this Scientific American link: The Origin of Oxygen in Earth's Atmosphere.
It's hard to keep oxygen molecules around, despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?
The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.
Which would mean a large percentage of the surface of your planet must be covered in photosynthesizing plants (algae, moss, plankton); otherwise all the highly reactive free oxygen in the atmosphere will collide with stuff (including other atmospheric gases and the ground) and be bound to it, there will be none left within a century (according to one source I read years ago but can no longer find).
However, recent studies show alternatives are possible:
The new research [by Norio Norita] shows that the interaction of titanium oxide [also called titania] with water could produce oxygen in the atmosphere of an exoplanet without the involvement of living organisms.
Fortunately for you, titanium is a low-conductivity metal; and the article goes on to say that on an earth-like planet, somewhere between 0.05% and 3.0% of the surface would need to be covered in the stuff to produce the same level of oxygen as we have on Earth (that would be 100,000 to 6,000,000 times the surface titanium dioxide found on Earth).
Titania must interact with water to produce free oxygen; I would recommend the water and titania be in deep reservoirs.
Killing the forests.
This is difficult to do on a temporary basis; I'd suggest orbital mechanics.
See this article on the formation of the Sahara: How Earth's Orbital Shift Shaped The Sahara.
The changes in the Earth’s orbital tilt and precession (or the wobbling motion) occur because of gravitational forces emanating from other bodies in the solar system. To understand exactly what happens, picture a spinning top when it is slightly disturbed. Just like a top, the Earth too wobbles slightly about its rotational axis. This tilt changes between roughly 22 and 25 degrees about every 41,000 years, while the precession varies on about a 26,000-year period. These cycles have been determined by astronomers and validated by geologists studying ocean sediment records.
So you have on Earth a 41,000 year cycle going on. This changes the insolation distribution of the Earth (yes that is spelled correctly, it is not "insulation", it is "insolation", which is a scientific term meaning the amount of solar radiation reaching the surface of a planet).
A minor change (half a degree) in orbital tilt turned a grassland into the Sahara desert by increasing the amount of sunlight to a point that plants simply couldn't handle it anymore; apparently the tipping point was about 6000 years ago (the approximate time frame you requested).
Your planet does not have to be Earth, of course; your cycle can be longer or shorter than 41,000 years: That is a peculiarity of our OTHER planets and their sizes and orbits. (you will need some other big planets to pull this off, to provide the necessary gravitational influences, but they don't have to be habitable, just like Earth's fellow planets are not habitable. I wouldn't bother trying to work out the math either, just that they exist and cause the axial tilt on a cycle: that's plausible.)
A second effect can be the presence of mercury-chloride and/or sodium-chloride (aka table salt) in large swaths of the planet, both of which are toxic enough to plants to cause desertification.
So the scenario is this: the South half of this damn planet is a natural desert due to salts. The North half can support plant life, but due to axial tilting in the orbit (due to resonances caused by some Jupiter sized gas giants in the system), on a cycle of about a hundred thousand years, the North half is exposed to extreme sunlight for a period of about ten thousand years. But that same axial tilting shifts underground water into titania deposits that react to produce oxygen; and this seeps up to sustain life on the planet. The plants have adapted, over billions of years, to produce seeds that can survive the drought. They won't sprout until the temperature has been hospitable for a few years, and the axial tilt has made the underground water available to them again. This is a cycle that has been going on for billions of years; and that is where the coal deposits come from.