Re: Conservation of energy
This very subject was discussed on something on Radio 4 a couple of weeks ago (sadly I don't recall the programme). Their conclusion was exactly as discussed above - save that the amount of energy involved was miniscule and significantly below the level at which it would be possible to measure any temperature changes.
That energy is stored in a spring is easy to demonstrate with an elastic band: stretch it and it will get hot - energy has been added to it. Let it cool to ambient and then relax it; it will cool further.