There is no infringement. You created the work, and there is no law that says it can't resemble someone else. As you say, political cartoons directly defame people all the time.
Site operators will always err on the side of caution since they don't want any chance of litigation or being shut down. You, on the other hand, have next to nothing to worry about concerning lawyers actually pursuing a lawsuit. They are just bothered that you might make money without them getting a cut, and want to make as much scary noise as possible.
Tell the lawyers to give me their names so I can demean their pathetic and worthless existence to all the world. I'll hire you to do the paintings.
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