Is the energy of matter (via $E=mc^2$) actually available to do work? If energy is functionally defined as the capacity to do work, then in what sense does mass offer a lot of energy (via $E=mc^2$). Most people responses I've seen to similar questions state “there’s a lot of potential energy available in the strong nuclear force”, but in the case of iron, it’s nucleus is the perfectly stable (no nuclear reactions can produce useful energy).
So in what sense does a kg of Iron have a massive capacity to do work? If it has to do with annihilation with antimatter, what is the force that brings about that change of state? It doesn't seem like it could be one of the fundamental forces.
 A: Any time mass is converted to energy - be it through chemical (yes! E=mc^2 applies to chemistry) or nuclear reactions or matter-antimatter annihilation, this energy can be used to do work. Usually it's in the form of photons, ie electromagnetic radiation, ie heat, or kinetic energy of a particle like a neutron flung from a nucleus at high speed.
In a nuclear power plant, nuclei undergo fission which releases energy per E=mc^2, in the form of fast moving neutrons, which in turn heat up water, which creates steam, which moves a turbine (work) which generates electricity (electromagnetism) which then can do more work.
This is true even for iron. E=mc^2 doesn't just apply to nuclear reactions or antimatter-matter annihilation. Whenever there is an exothermic chemical reaction, the end result is slightly less massive than the sum of the reactants' mass. The amount of energy released is equal to the mass difference times c^2. The energy released is photons, ie heat, which, of course, can do work.
Think of mass and energy like money in the form of two different currencies. E=mc^2 is the conversion rate. The fundamental forces are just different ways to spend the money.
A: An exothermic reaction decreases (rest) mass and increases kinetic energy, such as in a fusion or a fission reaction.  Look up the Q value for a nuclear reaction on the internet.  Also, an exothermic chemical reaction decreases (rest) mass thereby increasing kinetic energy but the change in rest mass is about a factor or 200 MeV/ 10 eV less than for a fission reaction, for example.
Endothermic reactions increase (rest) mass and decrease kinetic energy.
A: Energy does not solely manifest in ability to do work, chemically or otherwise.  A star or planet made solely of iron will generate a gravitational field because of the mass energy content inherent in it.
Also, if you bombard iron with antimatter as you suggest, the energy released will be partly converted from the rest energy of the iron nuclei as well as of the incoming antimatter particles.  Iron can still participate in exothermic chemical reactions such as rusting (oxidation), which decreases rest mass (though perhaps not much of the nucleus).  So its rest energy can still do work.
