Let us start with your basic assumption noted in your question, where you divide the human being into two; (1) the physical and (2) the mental, as if these could be separated in a person.  This dualistic perspective of being doesn’t agree with the biblical witness.  From a biblical point of view, which looks a humans holistically, you cannot separate the body from the mind or the mind from the body (Prov 4:23; 6:25; 12:25; 14:30; 15:13; 15:30; 17:22).  God created humans to be living beings, which entails body and mind (Gen 2:7).  The relationship of both body and mind is obvious to all of us; we daily live and move as our mind dictates and directs each part of our body.  So when we are addressing the question of what Ellen White calls “Self-abuse,” or masturbation, we must not think in terms of a merely physical act or neither just a mental act.  It’s really one act which entails the whole being, who is both physical and mental.  This is why Jesus could say in his sermon on the mount “that anyone who looks at a woman lustfully has already committed adultery with her in his heart” (Mat 5:28).  The sinful act of adultery, generally seen as a physical encounter, Jesus addresses as originating in the mind/heart, which then leads to the act.  It was already a sin once it was birthed in the mind; the physical action follows as an open sinful act.  Thus, to answer your question specifically, both the physical act and the thoughts involved in masturbation are a sin (Leviticus 15:14-16).  This is a quick summary of the biblical perspective on this issue.

Now, on Ellen White’s writing on this, we find that she complements the view stated above.  When she writes addressing the problem of masturbation or “self-abuse,” she always speaks of it in terms of being a sin that has both physical, mental and spiritual consequences.  Other than “self-abuse” (TSB 122), Ellen White calls masturbation a “secret vice” and a “corrupt habit” (CG 445-446); a “lust” (TSB 124), and “corrupt indulgence” (AM 27).

Her view is holistic, encompassing the effects on the whole person. Various illustrative statements are the following:

“Secret vice is the destroyer of high resolve, earnest endeavor, and strength of will to form a good religious character. All who have any true sense of what is embraced in being a Christian know that the followers of Christ are under obligation as His disciples to bring all their passions, their physical powers and mental faculties into perfect subordination to His will. Those who are controlled by their passions cannot be followers of Christ. They are too much devoted to the service of their master, the originator of every evil, to leave their corrupt habits and choose the service of Christ” (CG 445-446).

“The practice of secret habits surely destroys the vital forces of the system. All unnecessary vital action will be followed by corresponding depression. Among the young the vital capital, the brain, is so severely taxed at an early age that there is a deficiency and great exhaustion, which leaves the system exposed to disease of various kinds” (CG 444).

“I have been shown the danger of families that are of an excitable temperament, the animal predominating. Their children should not be allowed to make eggs their diet, for this kind of food—eggs and animal flesh—feeds and inflames the animal passions. This makes it very difficult for them to overcome the temptation to indulge in the sinful practice of self-abuse which in this age is almost universally practiced. This practice weakens the physical, mental, and moral powers and bars the way to everlasting life” (3SM 286).

“Some families were shown me as in a deplorable condition. Because of this debasing sin, they are where the truth of God cannot find access to heart or mind. This practice leads to deception, to falsehood, to licentious practices, and to the corrupting and polluting of other minds, even of very young children. The habit once formed is more difficult to overcome than the appetite for liquor or for tobacco” (3SM 286).

“Those who corrupt their own bodies cannot enjoy the favor of God until they sincerely repent, make an entire reform, and perfect holiness in the fear of the Lord” (AM 29).

“The only hope for those who practice vile habits is to forever leave them if they place any value upon health here and salvation hereafter. When these habits have been indulged in for quite a length of time, it requires a determined effort to resist temptation and refuse the corrupt indulgence” (AM 27).

As we can see by the above quoted statements, Ellen White viewed the practice of masturbation as a sinful act that has consequences upon the wellbeing of the whole person.  It corrupts both his physical and mental health to such an extent that it damages his spiritual outlook, undermining faith in God, and putting his personal salvation in jeopardy.  As believers in the Word of God, we cannot compartmentalize that which inspiration clearly speaks of as evil or sin and say that it’s acceptable in some circumstances.  In Ellen White’s words, “Sin, my brother, is sin” (TSB 124).

Now, I must make an observation regarding the quote you used from Ellen White’s writings.  It’s important that we read in context everything she says to correctly interpret her message.  Although Ellen White uses the phrase “self-abuse” in this statement that you quote from TSB, page 127, the major problem she is addressing here is what we would call today, child abuse, or more specifically, child molestation.  It’s within this context that she asserts that “God charges adultery against everyone who doeth these things” (TSB 127).  Furthermore, those involved in such evil “are leading youth into this habit of self abuse.”  Ellen White is unquestionably addressing a most important issue here.  But this statement is not addressing directly the problem of masturbation, which seems to be your main concern as I read it.

In conclusion, I would like to observe that I don’t find the two circumstances you describe as reasons that should compel any Christian to do anything else than to be faithful to God in our sexual life.  Life is made of choices, and we must choose the right course, requesting the aid of the Holy Spirit, and remembering the promising words from Scripture: “Therefore, in all things [Christ] had to be made like His brethren, that He might be a merciful and faithful High Priest in things pertaining to God, to make propitiation for the sins of the people.  For in that He Himself has suffered, being tempted, He is able to aid those who are tempted.” (Heb 2:17-18).  And also this , “For we do not have a High Priest who cannot sympathize with our weaknesses, but was in all points tempted as we are, yet without sin.  Let us therefore come boldly to the throne of grace, that we may obtain mercy and find grace to help in time of need” (Heb 4:15-16).

Thank you for contacting us. I hope this has answered your question.