1. A philosophy does not include dogma or ritual, a religion does. Satanism contains these things, and is therefore a religion.
2. See answer #1. Clearly religious.
3. Because Anton Szandor LaVey codified and in fact invented modern Satanism.
4. A defacto Satanist is a person who displays admirable Satanic characteristics but did not or does not take the name Satanist. A defacto Satanist is not something declared by Church decree, and should not be interpreted as an analog to canonization.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."