There is no problem to use coinjoin with a centralized service, it's all about being smart with how to use it efficiently.
How to do it has been explained so many times. If coinjoin was a real problem we would have already seen many more complaints on this subject. Even from people who never used it
Some of them do. It's more or less written on the user agreement. They have partnerships with blockchain forensic companies like CypherTrace, ChainAnalytic, AMLbot, and so on. It's not really something secret. Some brag about their partnership on Twitter and co.
It's true but CEXs have a few advantages that DEXs don't have. Also, you may want to use a CEX because you want to buy