So what? Here in America we have celebrities pontificating all the time on things they know nothing about, influencing possibly millions of people. That doesn't mean they are factually correct.
You say you're disinclined to believe him compared to other witnesses because he may have political bias. We've shown that no Swedish submarines existed that could have inflicted the damage observed on the starboard side of the ship. Yet for some reason you seem to think his claims still merit attention. Why are you reporting things you don't believe, and which can't be true? Why are you compelled to defend him when his testimony is challenged?
I know the difference, but it's not clear that you do. You constantly vacillate between the motte argument that we have to pay attention to him because he allegedly has insider knowledge or that he's a well-known figure, and the bailey argument that he's just a political hack and you're reporting what he says out of general interest, and you shouldn't be held accountable for what he says, or to reconcile it with your other reporting.
The problem with most conspiracy arguments -- yours in particular -- is that while real investigation and real journalism are trying to go toward any of several possible conclusions, conspiracism is all about running away from one conventional narrative. If you declare the conventional narrative the thing that you must abandon as a premise to your own study, then there are infinite directions in which to run away from it. And many conspiracy theories -- yours especially -- try to go in all those directions at once. As such they fail to provide a compelling alternative narrative. Just because you conclude that the conventional narrative is flawed doesn't mean you can't also conclude that some of the alternative narratives are also flawed and should be abandoned for the same reasons you abandoned the conventional one. Because in the end you have to provide a single coherent narrative that stands against the conventional one, and is sensibly more supported by evidence. Clinging to any and all stories that dispute the conventional narrative accomplishes nothing.
In a real investigation Kurm's statements would have been rejected long ago and never thought of again. In a conspiracy theory, they keep popping up only because they dispute the conventional narrative (i.e., they support the running-away mode), not because they contribute to any better understanding of what might actually have happened.