i am fairly certain you are wrong on this. Iran is another example of a country US firms are banned from doing business in. One US oil company was until recently doing maintenance work on an offshore facility owned by iran from a pre-revolution contract, but no one can do anything with iran proper. mark rich got into his bind over that, and had to pay off the clinton team to get a pardon much later.
look at all the european investment in foreign-tourist areas/hotels of cuba. there is a reason why no US company is there, and it isn't patriotism.
Depends on where a corporation is chartered. If a multinational is chartered in a country that doesn't participate in the embargo, Cuba may allow foreign investments. I don't think Cuba allows foreign investment in sugar & tobacco.
I don't know what Peter Paul's FReeper handle is, but he got mixed up in some Cuban trading... something involving coffee. I read something at FR with the details, but I can't remember the details of it anymore.