The market value is not calculated on the basis of the cash that a person has or how much money a company has in the safe. The market value is based on estimated calculations, which can often be distorted due to political reasons or the stock market speculation itself. Either way, there is no way that Elon Musk can physically shake the price of Ethereum with his money.
Yes, that is true. Net worth is what is owned minus what is owed. Since we don't know all the figures involved, the market is just giving the rough estimate of the net worth of a person. We don't know all the assets and liabilities of the person even if we say he is a public personality. Not all are disclosed to the public. And do you think, for example, Elon Musk would like to bet all his money to one project? I don't think so. Businessman tends to diversify their portfolio, so they won't allot their wealth into one asset only.