Most autoerotic practices are relatively or entirely safe.
One example of a dangerous practice is autoerotic asphyxiation. It often results in death, particularly when done alone.
Self-bondage is the practice of sexual bondage by oneself; it carries a higher risk than bondage with a partner, and there is a large potential for things to go wrong.
In so doing, my intent is not to condemn nor vindicate, but to uphold a view that solitary sex (autoerotism) promotes a narcissistic view of sex and robs a person of what sex can and ought to be both for the self and for whomever the person may enter marital covenant with.
Fundamental to the morality of autoerotism are two issues: ownership of the body, and the purpose of sex.
When you say things like "Can it truthfully be said that autoerotism brings one closer to God, or, at the very least, does not separate one from God, if it indeed represents a use of sex for which it was not designed," you are trying to redefine sex as sexuality, and that is dishonest.
His Sexual Inversion, the first English medical text book on homosexuality, co-authored with John Addington Symonds, described the sexual relations of homosexual men, something that Ellis did not consider to be a disease, immoral, or a crime; a bookseller was prosecuted in 1897 for stocking it.
Other psychologically important concepts developed by Ellis include autoerotism and narcissism, both of which were later taken on by Sigmund Freud.
Ellis was a supporter of eugenics which he wrote about in The Task of Social Hygiene.