The Malaccan Straits is shared by two nations, but international law recognizes up to a 5 mile territorial limit for coastal waters. Should Malaysia and Indonesia ever unite, then the Straits would belong to a single nation and could be legally denied. If that, we'd just go around. Until then, like the Persian Gulf, it is open sea. Open seas are always free for all shipping. For any nation to prevent that is either an act of war or piracy...I have no problem defending our rights to sail the open seas.
I think I've answered your "trade" question by my responses above.