Um,probably need an expert here, but my understanding is that the Supreme Court has said that once you are not permitted, or reasonably believe you are not permitted to walk away from the police, you are under arrest. Period. Thus, putting you in handcuffs means you are not free to leave and are under arrest, no matter what they say. (they are allowed to lie, remember)

Once you are under arrest, all of your constitutional rights are in force, e.g. right to silence, right to council etc.

So, no, as far as I understand, detaining you for 48 hours for questioning is an arrest.

And if asserting your rights makes them suspicious, so be it. I'd rather that then say something stupid that will make me look guilty later in Court.

In your scenario, they would need probable cause to arrest and hold you. Your scenario makes it clear that there is some evidence that you could have committed the crime. That (they would argue) is their probable cause for the arrest: that you committed the crime. They cannot arrest you for not answering their questions. You are under no legal obligation to respond. In some cases, if you do choose to respond, you can be criminally liable if you lie or deceive with your answers. But if you witnessed a crime and simply don't want to answer and walk away, you are not committing a crime and they can't arrest and hold you "for questioning."
_________________________
"Better is the enemy of good enough."