Lansky may have been the most influential Jewish gangster of the 20th Century, but he wasn't the richest or most powerful. Lansky was reputed to be "worth $300 million," and that stuck with him forever. But he was one of the few gangsters who had a competent biographer. Robert Lacey, in his superb "Little Man-Meyer Lansky and the Gangster Life," tracked down the $300 million figure to author Hank Messick, who said he heard it secondhand and used it because it was a sensational figure. In reality, says Lacey, Lansky at his peak was worth no more than $5-6 million--not chickenfeed, but hardly the stuff of $300 million. Lacey says Lansky lived to 81 and died peacefully because he never had either the power or the money to attract jealousy, resentment or greed of other mobsters, which invariably leads to murder. "He was the accountant, not the boss."

The wealthiest and most powerful Jewish gangster of the 20th Century (in terms of the reach of his criminal interests) was Louis Lepke Buchalter, followed by Abner (Longie) Zwillman and Dalitz.

Unless I missed something or you left something out in the quote you provided, I don't see a connection between the McDonald murder/suicide and Siegel's assassination.