First - make a point by letting him go. Second - revise the existing legislation to benefit the people involved. That means, if the military ****s up because they are witholding information that would be better off revealed, then the whistleblower can live his free life. If the whistleblower reveals information that legitimately causes a threat to national security - then AFTER proving as much - they can choose to put the individual in confinement.
This action will support and promote vigilantism. Manning acted as judge, jury and executioner all based on his subjective interpretation of the situation.
As far as whistle blower issues are concerned.. First, I agree and support that ideal, however, there must be a structure in place to facilitate the process. Manning had a number of options he could have pursued once he exhausted internal channels including releasing these docs to a Senator or House Representative... Instead, he released them to an entity that maintains a sole directive of embarrassing/disrupting national gvts.
As far as the 'withholding of info', Manning released as many documents as he could on a carte blance basis, there was no selectivity in the documents that he forwarded to wikileaks.
This would help the system focus on the issue at hand - which is the consequence of revealing pertinent information. Not the act of revealing information itself. If there is no harm to public safety - indeed in this case there is benefit - to revealing that information, then the whistleblower should not be punished for the sake of queen and country's reputation.
As mentioned, there needs to be internal options made available (first) and alternate options in terms of gvt (considering that the military is an arm of gvt).
None the less, you have hit on possibly the most important point: "If there is no harm to public safety"... Manning had no clue what the fallout would/will be and it is more than likely that the animosity between nations that these docs have generated will result in doors being closed to programs that may have benefited high needs groups.
With that in mind, I'd suggest that Manning's actions did and will cause harm.