For a single traditional Catholic, to avoid the possibility of scandal, unless it is demonstrably easily provable that the person who you are intending to marry has without a question of a doubt a true case of an annulment (never consummated the marriage etc...). Then it would still be licit, but as a general rule simply take out of your mind that possibility of marriage with that person. Or if you are patient enough wait until other spouse dies, lol. Feed the other guy a diet of McDonalds, ahahah. On a more serious note, things are so difficult at the moment. Any state of life you choose, is riddled with so many obstacles so do not be faint of heart and simply have faith that God will provide.
Actually, it is the tribunal which determines where the scandal, if any, lies. If they determined the danger of public scandal was insignificant, they would be bound to order immediate separation from bed and board in order to avoid sin and the near occasion of sin for both parties.
If they determined the danger of public scandal was significant, they would still be bound to order separation from bed, the parties would then need to strive to separate from board also, but under close monitoring and council from the tribunal.
While there probably are those cases where "living as brother and sister" would be permitted for a prolonged period, that would be the rare exception and would be due to extraordinary circuмstances, most likely involving young children in some way.
Beyond that, the Church tribunal's primary duty is to safeguard, preserve and defend the sanctity, integrity and dignity of the sacrament - and of the Marriage Contract. That is what it is all supposed to - and used to revolve around.
Living as brother and sister would only be permitted if it first determined through a thorough investigation of all the circuмstances, that no scandal would befall the integrity of the sacrament due to the public thinking "the Church grants divorces willy nilly" like they do now.