That has nothing to do with the point you were making. You were talking about the benefits of interviewing early to trip the suspect up on their own story. They already have that story, so what difference would a video that doesn't show CB in it make to that alibi?
"HCW: Excuse me Herr Brueckner, did you make this video?
CB: No.
HCW: Oh well, worth a try. Here's all the other evidence we've got on you. Pick your own version of events based on where our holes are."
They force him to explain where he got it from. That's the point. Get the suspect on the record, with things you might be able to disprove for trial.
How does he? He would know there's a video, sure. He doesn't necessarily know they have it.
The hypothetical is the video is amongst those recovered at the factory. If it exists, then he knows they have it.
Plus we're just talking about the big evidence here that proves MM's death. HCW has said there are lots of pieces to the puzzle that point to CB. He would have to hand over all that evidence too if they were to quesion him.
I am not so sure about that. In the UK and NZ, they can question him only about the video. So long as they don't charge they would not have to disclose the rest of the case.
I really don't see the benefit of showing their hand while he's not got a chance to run anywhere. Just keep building the evidence for now. Your argument really doesn't make much sense IMO. Tell me what they stand to gain as opposed to waiting for more evidence first?
The only reason normally to keep evidence secret, is to prevent the accused from perverting the course of justice by destroying evidence etc. In this case it is hard to imagine what advantage the defence could gain.