Point by point....
Second Hague Convention Article 29 (I assume you mean Article IV, you don't say and it is important):
First off, these conventions are intended for warring States. There is no provision covering international movements or other extra-governmental organizations. So Wikileaks is not a protected or described participant here, and neither are groups which do not represent a territorial collective will, such as Al Qaeda. So this does not apply, as there is no State of Wikileaks.
Even if it did apply, Manning was bound by restriction of his duty and the agreements he had signed to only pursue information he had a "legitimate need to know". Downloading every document you can possibly touch is not bound by any "need to know", and therefore was indeed done with false pretense.
Article 31:
This assumes the other State had captured Manning as a normal battlefield soldier, not as a spy. Since Wikileaks did not capture Manning on the field of battle and seek to execute him as a spy, this does not apply. Since Manning is not a part of the Wikileaks National Defense Force (lol) and captured by the US Army on the field of battle after having spied on the US and returning to uniformed military duty with the WNDF, this does not apply.
The Nurenberg Trial principles and Red Cross Conventions you quote are just nonsense to try to create a strawman. This is not a battlefield, and Manning is not even an ersatz medic.
Manning was a US Soldier, caught by his own country, under laws which are applicable to him by his citizenship. And in no case does international law supersede national laws in a matter such as this.
Wikipedia puts it simply:
United States Code at 18 U.S.C. 2381 states "whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere, is guilty of treason and shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United State
There is no argument which can countermand this. International accords about the behaviour of States under a declaration of War do not apply, as there is no State involved in Manning's situation. But there are enemies.
Manning is guilty of Treason under US laws, and deserves the associated penalty.