Why did Jesus not suffer eternal separation from God after he BECAME sin for us?
ANSWER: A few things need covering first before we arrive at an answer. First, is "death" in Romans 6:23 the same as "conscious separation" as is taught by the commentaries you mentioned? This is what far too many 'Christian' churches read into the verse. Their interpretation is primary based on the false belief in a conscious or 'immortal soul' that continues to live after death, a soul that could immediately experience the flames of Hell and begin to suffer eternal separation from God.
The Biblical definition of death is that it is a cessation of consciousness, something akin to someone who is asleep (see 1Corinthians 15:20, 1Thessalonians 4:15 - 16, John 11:11 - 15). Ecclesiastes 9:5 - 6, 10, states that the dead know nothing and are not to be able to do anything. Immortality is conditional (we do not already possess it as the rest of Romans 6:23 implies), and people do not go to heaven or hell right at death but are simply dead (unconscious) until a resurrection. This means that Romans 6:23 cannot be rightly used to justify a possible conscious and eternal separation from God occurring right after a person's physical death.
A discussion regarding the commonly held Christian beliefs in an immortal soul and in an ever-burning Hell of torment is beyond the scope of this short answer. Please see our article "What does the Bible say about HELL and PURGATORY?" and "Do we have an immortal SOUL?" for more information.
Jesus, up to the time he died on the cross, had ALWAYS existed with God the Father since he was also deity. Because he was (and is) God (and the actual Creator of the universe and man, see John 1:2 - 3, 10) His life was worth far greater than ALL other life (human, angelic, etc.) that had or would ever be brought into existence.
Jesus paid the full price for ALL sin (past and future) by living a SINLESS life in the flesh and subjecting himself to suffering and an ignominious death he did not deserve. He willingly allowed his perfect, eternal life to be ended (for a short time) when God made him bear and become the embodiment of disobedience. Jesus then tasted the same kind of unconscious existence all humans experience when they die. After fully paying for all our transgressions and making possible man's salvation by a complete reconciliation to God upon repentance, the Father resurrected from the dead.
Soldiers playing games at Crucifixion
An even greater question lurks within the one mentioned in your Email, which is the following. WHY did the two Beings of the Godhead decide, at some point in the eternal past, that a sacrifice for sin would be given after it entered the universe? Why was it not decided to offer forgiveness upon repentance without including a sacrifice? The answer to this most profound question has been a topic of debate among scholars and believers alike for many years. Although BibleStudy.org does not claim to have the complete answer to this inquiry, it offers the following to consider.
Jesus told his disciples, just hours before his arrest and crucifixion, "No one has greater love than this: that one lay down his life for his friends" (John 5:13). It was ultimately decided that Christ had to die as a sacrifice for sin because it would be not only the most powerful demonstration of sin's consequences but also the perfect proof of God's eternal and perfect love for his creation.