Be prepared when you return from Cuba to be interrogated for possibly multiple hours by US border patrol. That's what happened to me. If they ask you why you went, do not say "tourism." You have to say, "support for the Cuban people." Everything will be easier if you go through a travel agency, as they will understand all the rules regarding the sanctions. I believe travel one does trips to Cuba. As far as getting interrogated, things to remember: You have to give them your name, your ofac license, passport, where you stayed, and your bags. They are allowed to search all of your bags without warrants, so do not bring back contraband. Pretty much the only things you should bring back are books and art pieces (and they may take those). You don't need to answer any questions being those that they ask, but they also do not need to provide you a lawyer and can detain you for almost as long as they want. HOWEVER, if they cause you to miss a connection, they are required to give you a new flight. I could talk more about my experience but don't have time. DM me if your want to talk more.
Don't know much about faking drug tests. However, if you need to buy yourself some time, you could fake a positive COVID test the day before. That way you could delay taking your drug test until later, giving you more time to detox or to acquire a fake urine kit.
Faking a positive COVID test is pretty easy for a rapid test, you literally just put anything acidic on the swab.