I'd think that if a cop followed the law and department policy he'd be personally exempt from liability. I'd even add some leeway so that mistakes and snap misjudgments don't remove that exemption. However, I am for personal liability for willful violation of a person's civil rights. This is a good example, since the cop had no reason under law or policy to perform an arrest, and it wasn't a stressful situation calling for a snap judgment.
I think there could be a way to make it work. Like the do in the miltary...every year I have to have sexual harrassment training (no not how to do it, but what you aren’t allowed to do) along with various other types of training like law of armed conflict.
So if I do violate the sexual harrassment policy or law of armed conflict then I can be tried in court. So possibly if law enforcement got routine briefings on this stuff then yes I concur it would be ok to nail them.
What's needed is a recognition that actions contrary to the Supreme Law of the Land are illegitimate(*), that illegitimate actions form no part of any government agent's legitimate duties, and the generally-broad immunity officers enjoy when performing their legitimate duties does not legitimately apply when they are acting illegitimately. Recognition of those points would help a lot.
(*) With the caveat that unconstitutional actions may sometimes require extra-constitutional remedies. For reasons somewhat analogous to the mathematical Incompleteness Theorem, no constitutional system of government can fully handle all cases where people try to circumvent it.