Approaching the Qur’an without reference to the Sunnah is wrong. Allah did not send His messenger only to deliver the Book as if he were a delivery man who is not interested in the contents of what he delivers. Rather, Allah tells us in no uncertain terms that He did send down the Book to the Prophet (peace be upon him) so that he expounds the message to the people.
“And upon you [too] have We bestowed from on high this Reminder (i.e. the Qur’an), so that you might explain to the people all that has been sent down to you as a message for them that they might heed the lessons.” (Qur’an 16:44)
Therefore, the companions, as well as all of the succeeding generations, looked at the Sunnah as the infallible interpreter of the Qur’an. Therefore, for anyone to claim that he can dispense with the Sunnah in understanding the Qur’an he is simply self-deluded; he is guilty of making up a new religion.

Obedience to the Prophet (peace be upon him) is integral to obedience to Allah: “Say [O Prophet]: “If you love God, follow me, [and] God will love you and forgive you your sins; for God is much-forgiving, a dispenser of grace.” (Qur’an 3:31)

Having said this, however, I do not mean to say that we must accept each and every report found in the collections of hadith as if they were integral aspects of sunnah to override the plain meanings of the Qur’an. For sure, as shown by the companions like `Umar, `Aishah, and others, we ought to disregard the doubtful and dubious traditions when faced with incontrovertible evidence or principles of the Qur’an.
The issue of Rajm you mentioned has been approached this way by some of the eminent scholars such as the late Shaykh Abu Zahrah; however, this view is not widely accepted by other scholars.
It is true there is no mention of Rajm is not found in the Qur’an; so we may take it to mean that it is not the standard punishment enforced for Zina at all times. However, we cannot deny Rajm since it has precedents in the practice of the Prophet and the pious caliphs.
Having said this, we ought to point out that Rajm is more like a deterrent than a punishment as the conditions of enforcing it are at best extremely rigorous and therefore next to impossible to implement it. The Prophet (peace be upon him) has already instituted the principle: “Avert punishments when and where there is a modicum of doubt.”
Therefore, I would agree with those who call for a moratorium on such corporal punishments as called by Dr. Tariq Ramadan and others. By doing so, we are not in any way tampering with the Shari’ah; rather we are enforcing it by looking at its primary objectives and purposes.
We have numerous precedents for imposing moratorium such as the following: Hudud are not to be implemented during the state of war; Umar’s refusal to enforce cutting off hands during famine and starvation, etc.
As for the killing of apostates, we have reasons to believe that it was implemented only in cases involving treason. In other words, neither the Prophet nor the pious caliphs ordered hunting down those who left Islam – for exercising their freedom of conscience.   Freedom of conscience is one of the fundamental tenets of the Qur’an reinforced through dozens of verses; hence we cannot dispense with it.
Mind you the soul and spirit of Shari’ah is justice, compassion, and mercy.
Allah knows the truth.