If there is no verdict issued, then it is such that he is, by definition, innocent. The fact that some attribute a 'we don't really know' approach is precisely why doubt should work in his favor, especially when the police did not believe the evidence was sufficient for a conviction.
This is the core of the discussion regarding Greenwood's return, so we need to address that.
But yes, one can be guilty without being convicted. However, the point remains that knowledge of whether a person is guilty is crucial for a verdict. If a murder cannot be solved due to a lack of concrete 'knowledge,' you can claim the person is guilty, but it's not based on objective findings.
Legally speaking, Greenwood is innocent! Alternatively, we can all individually speculate on what he exactly did - and in that way, attribute guilt to him for actions that would even vary if you ask 10 different people.