I did the defender of light and when I went into the place after I beat the game there was 1 guy there and a BUNCH of monsters plus a rather annoying banshee. I wanted to buy it but I don't think I can.
You cannot own both temples at once. The quest Defender of the Light\Oakfield determines which temple will be available for purchase after you come back from The Spire.
Failure to do Defender of the Light will always destroy the Temple of Light.
This is true. If you do the Oakfeild massacre quest then the Temple is destroyed. If you defend the light then it's saved and the Temple of Shadows is destroyed instead. However, if you do neither the Oakfeild massacre still takes place just without you there and thus the Temple of Light is destroyed if no action is taken.
We're all going to die some day, I intend to deserve it.