1) You can do the SoP twice a day, as long as you don't find that it makes you feel spaced out.

2) The animals aren't fixed -- they're specific to the modern Druid Revival tradition. You can replace them with other animals, or with other beings -- for example, many Christians invoke the archangels in this same place.

3) That's correct; Earth is the topsoil and what grows on it, Spirit Below is the force that moves through the deep places of the earth. A chthonic goddess would be a good choice.