Sure, that would be the case with you and me, not being royal. But, doesn't she have immunity because she is a member of the royal family. I know that even diplomats have immunity when it comes to crimes, which IMO is very controversial.
Can she be arrested and interrogated being the CP and future Queen? I don't know royal or Norwegian laws, other members here probably know better.
I'd say on a practical level, it would up to the King to decide whether MM should be questioned. In theory he could conduct such a questioning himself (or more likely his lawyers could, or someone appointed by the King, say a Supreme Court judge).
I dare claim that the government and by extension the Parliament can "persuade" the King to let the Crown Princess be questioned.
But of course MM could on her free will let herself be questioned by the police - in order to assist the police with their investigations.
In any way, if there is a demand for questioning MM and it is vocal or official enough, I find it difficult to see how the NRF can refuse it, without another very serious dent in its reputation. Because the signal would be that the NRF, led by the King, consider itself above the law. It would also very likely be interpreted as an indirect admission of guilt.
It is really a Catch 22.
Marius could actually get his mother off the hook by simply admitting that
he destroyed the phone and SIM card. And unless he knew there was a warrant on his phone
prior to his arrest, I suppose he could get away with it. After all there is no law against destroying your phone if you feel like it. - Alternatively take the rap for destroying potential evidence himself.
But from what I have learned about Marius in this thread I'm not sure he would do that.