Can I find a clear description somewhere of how the available power, when an active medium is used as an amplifier, is related to the maximum power you can extract from the same system used as an oscillator?
That cannot be answered simply. There is not any fixed rule saying you can extract X of the stored energy in an oscillator and Y in an amplifier. There is not even a tendency. What you get depends on a variety of parameters of how you use it.
I can recommend “Solid State Laser Engineering” by Walter Koechner as a compendium of knowledge of laser design and modeling.
The available powers should be slightly different, affected by the seed laser in power amplifier. The difference could be found in “Solid State Laser Engineering” by Walter Koechner.