Well, “probable cause” certainly is typically a question of mixed fact and law.
However, the narrow question here addressed is: is an illegal arrest, illegal because the officers failed to undertake the formalities required to make an arrest, a mistake of law or a mistake of fact?
I would state that, if it is based on a mistake (or deliberate error) of law, it is an “assault” - and fits more within the principles of * R. v. Plummer* than those of Bivens (ignoring, for the sake of argument, that they are from different jurisdictions).
This is because it is a general legal principle that mistake of law is typically not a defence (though mistake of fact can be).
In the various situations described upthread (and indeed this debate) grew out of the following proposition: that in the State of New York (as in many other places), there exist certain formalities required to secure a legal arrest: notably, that the person must be informed of the reasons why he or she is under arrest. It was posed that, should these formalities not be complied with, there would be no consequences.
Not that the arrest was mistaken because the police lacked “probable cause”, but that the arrest was not legal because the requirements for a “legal arrest” were not followed (either through mistake or otherwise).
Ignoring again for the sake of argument that the formalities may well have been complied with “off tape” (we simply do not know), what would be the legal status?
We know in New York that resisting arrest would still be an offence, because the statue makes it so. However, absent some equivalent statutory intervention concerning the defences available to police officers (which may well exist - I have not looked), the police officers would have committed an “assault” (though obviously whether any charges would be laid, or damages sough for torts or for violations of rights, would very much depend on the circumstances).
Again, as far as I can see, where an arrest was “not legal” in this sense, a finding of assault follows - as its legality in these cases is a matter of law and not fact or mixed fact and law (other than the “fact” of whether or not the formalities were complied with).