"it is hard to believe it lawfully purchased. "
Why, becasue he was a French imperial maréchal and thus "bad" ? I would have thought a nice rich grandee of an Empire could have afforded to pay for things, especially if war had driven down art prices. But, he was accused of looting, not benefitting from others' distressed circumstances.

"the plunder of the Revolutionary wars"
Frankly, I have not studied the era of the Révolution enough to even ask an intelligent question about those years. The topic, however, was the Empire.

"Napoleon himself assumed his generals were stealing"
Maybe, maybe not. I would love to see the quotes that indicate this.
But, whatever his assumptions, we still have three "problems" :
(i) identifying those actually guilty
(ii) identifying what they stole and from whom they stole
(iii) determining if the amount in question was so trivially small that it had no economic impact - therein including the emulation by others in the army of the actions of the généraux

Artwork, I will admit, does not require item (iii), as it goes to questions of cultural patrimony or national identity (not that anyone really thought in those terms about artwork at the time, but they could have).

Anyway, I eagerly await your further research on this topic.