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."