There's only one thing that could really put Luca's position as a regressor in question: treating Ruediger so distantly from frame 1.
If he was the Luca of the original novel, he might've kept some positive memories about his uncle and not started immediately antagonistic. But he has always, at every point, kept him at a distance, never once showing true warmth toward him. Everything else is explainable as him being a regressor.