dimorphism
Yes - its a voluntary interview, so she can decline to be interviewed or walk out of the interview at the start if she wants - that's perfectly fine
But the police will only have the complainants side of the story to go on. So it will end up in court as one word against nothing.
This is probably why her solicitor may have suggested to her to be interviewed.
Unless there is evidence to suggest she is leaving for Australia for good - they can arrange a vol interview when she comes back - so its not being done so she can leave the country. Since the incident happened in September, which ever cop is dealing with it doesn't see it as a priority and is probably busy dealing with a whole host of other crimes and ongoing emergencies which has pushed this on the back burner. A couple more weeks won't make much of a difference.
These multiple videos, and witness testimonies and police statements you mention - where are they? Can you give me a link which shows the event from start to finish giving 100% continuity of KJK on film? Can you give me a link to the witness statements from police and other witnesses at the event which support her account? If not - how can you be certain they exist and what they say.
And yes - you don't need to be interviewed to be charged. But to give a fair and open investigation - they should be given an opportunity to be interviewed at least.
The bike issue - complain if he was adamant that it was one and same bike. You will be able to access the crime investigation log to see why they made the decision not peruse the allegation. Can he say 100% that it was the same bike for example?
Yes - Couzens should have investigated for flashing. Did the people being flashed at identify him and provided a statement at the time? If so, yes he should have been prosecuted if the evidence was there.