The murder was carried out with a gun - as far as you know, Jean Jacques always carries a weapon with him. Your father is a desperate collector of rare things. He could easily kill for an antique.
The name of another possible culprit is Salma. You don't fall for the image of a quiet wife who is fine with her husband's infidelity. This woman has a strong spirit - the mansion's relative peace and quiet is the result of her strict master grip.
You think that the real motive for the crime was the text of the will. This is pretty obvious