I appreciate your point of view and maybe I won't change your mind, but will provide more about Hoppe v Klapperich:
“If the attorney proceeds upon facts stated to him by his client, believing those facts to be true, and if those facts, if true, would constitute probable cause for instituting such a prosecution, then the attorney is exonerated.”
Hoppe v. Klapperich, 224 Minn. 224, 242, 28 N.W.2d 780, 792 (1947).
If the client's claim was true, he paid a stranger in a sauna $850 in cash for a photo to use in advertising. He admits having no personal knowledge if the stranger actually owned the rights to the photo. I said was my photo, and was used without permission. They initially agreed with me...until I posted a web page. Then they reversed positions, said the stranger was the true photographer, and sued for defamation.
I had a certifcate of copyright registration, proof of prior publication, out-takes, the high-resolution file. I offered this evidence, they said they didn't want it.
Not wanting to pay the rightful owner when you are using stolen property is not probable cause to sue the rightful owner for defamation. That means the attorney is liable (under Hoppe) because the litigation lacked probable cause.
It might be akin to suing someone for defamation because they claim to own the house you are squatting in. They have title to the house, all the neighbors agree they have lived there for years, but you claim "I paid a stranger in a sauna $850 to live in this house". Probable cause does not allow for the absurd, and I believe such a claim is absurd.