most (not all, some contributions are QTVR-only due to historical reasons)
many, but depends on photographer's choice of file upload (newer event = more likely to work) and your device's capabilities. You may experience slow playback, distorted images or unexpected browser quitting if there is only limited memory/GPU power available.
Equipment: I took four shots using a Sigma 8 mm fisheye lens connected to a Canon EOS-7D body. The angle of the camera was around +30 degrees, which gave me a hole at the Nadir but I didn’t need to work on the Zenith because PTGui Pro took care of that for me.
I then processed the image in CubicConverter. I converted the file to a Cubic QuickTime Movie and then exported the bottom face to Photoshop where I fixed the Nadir.
Back in CubicConverter I replaced the fixed bottom face and converted the resulting QuickTime Movie to a equirectangular Image.