You seem to underestimate the trade and links between the U.S and other major world powers. Some billions dollars oil deals/investments in Iran is peanuts compared to the trade,links, interests that E.U, China,India, Japan, and other world powers have with the U .S. Iran is still relatively closed to foreign investors anyway, unlike say Gulf states who have a level playing field for foreign investor and among the best in terms of ease of doing business/attracting investors. These are things Iran has to sort out in the first place irrespective of sanctions or not. Since even without sanctions , there are barriers for foreign investors in Iran be it politically, financially, or in terms of laws/policies.
So if U.S confess goes ahead with re-imposing sanctions on Iran and any company involved with Iran, then believe me these companies/countries would have to follow or limit their involvement with Iran. So the thing here should be to hope Congress doesn't go ahead with sanctions.
Sarcasm or?