Christiaan has answered your first question, it is external constraints that restore the muscle `rest' shape — this is also the case for nonskeletal muscle as the bladder.
On question 2, I will give a little more details, broadly based on Huxley 1957: imagine the muscle is trying to contract against something infinitely resistive. Full contraction cannot be achieved. Myosin heads still bind to actin, but actin cannot slide wrt myosin filament, and only a local deformation is possible under this tension increment. But myosin heads don't stay bound for ever, and when one detaches, this tension increment is lost locally (there'll be heat produced, dissipating this energy), thus the global muscle tension doesn't increase further. This is a built-in limitation rather than a question of how much energy is available.