If Sheppard had acted without the prior knowledge of the SGC, he would have been held accountable and be awaiting disciplinary action. He would not have returned to Atlantis. A court martial would be far to public and involved for the situation. Sheppard could have been quietly and simply dealt with in several ways. He could have found himself in a cell in the SGC, dishonorably discharged, back in Antarctica, or, if his experience was considered too invaluable to lose, back on Atlantis, demoted or with a loss of pay and privileges.
It would be difficult to bring any charges against Sheppard (they have to be secret) without first talking to his commanding officer, in this case, probably, Landry, to find out his reason for accepting the report. In fact, no charges would have been brought until all the principles had been interviewed and there had been an investigation of the circumstances and a recommendation of what charges to bring and who should be charged.
Wallace’s death was, however, facilitated. There is no other scenario that can explain what happened. Wallace walked to his death on his own. Sheppard took him to the lab, unchained the Wraith, made d@mn sure that the guards would not try to save him and stood by while the Wraith killed Wallace. What Sheppard said to Wallace in inconsequential and immaterial to the actual act of facilitating his suicide.
What Sheppard said to Wallace was very carefully worded (by the writers). There is nothing in the conversation but the basic facts of the situation. There is intentionally nothing in the wording that can be specifically construed as ‘pressure.’ There is no mention of feeding the Wraith, no wording that indicates that Sheppard would be willing to feed anyone to the Wraith and certainly no overt encouragement for Wallace to volunteer to be fed to the Wraith.