(This is point number 3) I'm not sure about what you mean with "source of photons from nothing": you can't create energy, so for example you need to provide the energy needed to your quantum system for going to $E_2$ first. Said that, the photon wasn't "inside" the system, it was created, 'from nothing' if you want.

 1. See the comment for lower *energy* limit. For an upper limit, if you give too much energy to your oscillator (how much depends on the system) you'll end up destroying it (tearing apart its costituents), so any theoretical upper limit (see the question you linked) is not very relevant.

 2. Yes, for example using Compton backscattering [en.wikipedia.org/wiki/Compton_scattering][1]. Notice that there you don't create photons directly, but instead accelerate already existent photons.
 


  [1]: http://en.wikipedia.org/wiki/Compton_scattering