What is the ruling on working in marketing for centers and hospitals that deal partially with commercial insurance? And will I be burdened with the sin of everyone who goes to them through me and uses their commercial insurance? And must I investigate the reality of the cooperative insurance companies that these centers and hospitals deal with, or is it sufficient for me to rely on what is apparent?

It is permissible to market for centers and hospitals whose work is permissible, and to receive a fee or commission for that, because it is a permissible benefit. As for clients paying fees through commercial insurance, this concerns the clients themselves and not the marketer. It is permissible to act based on what is apparent until proven otherwise, as long as the matter is open to interpretation. Selling juice to someone who is believed to be making wine from it is forbidden, but the sale is permissible if the matter is open to interpretation, such as when it is bought by someone whose condition is unknown, or by someone who makes both vinegar and wine and has not explicitly stated an intention to make wine. Dealing with someone whose money is questionable or mixed with usury is not forbidden unless it is certain that the money itself is unlawful or derived from unlawful sources. The default assumption regarding what is in a Muslim's possession is that it belongs to them if they claim it as their own.