I agree that the overnight attempt by the police to gather further information at the roundabout would suggest that they need more evidence prior to making an arrest(s).
I know that everyone would like to see this one wrapped up quickly, but sometimes these things take time. Even the analysis of evidence that they have already collected, and might be able to use to build into a stronger case, is likely to take some time.
There is an ongoing case in WA (Corryn Rayney) that will come to trial in July where the husband was named early on by the police as the "prime and only suspect". It took nearly 2.5 years for the police to collate enough evidence to charge the husband.
Furthermore, the husband (who is a lawyer) sued the police for defamation for naming him as the "prime and only suspect". I would expect that all law enforcement officials (especially in Australia) have been a bit more cautious about naming persons of interest and suspects in murder cases as a result of what happened with this case.