What is the ruling on lying to a suitor about a past relationship for fear of separation and scandal, and is it permissible to conceal the details of the relationship from him if he swears by a vow of prohibition (tahreem) should something warranting it have occurred?

You must repent from the forbidden relationship and not inform the suitor about it, as that constitutes publicizing a sin. The suitor has no right to ask the fiancée about a sin between her and Allah, and she is not at fault if she cannot دفع it except by lying. If the suitor conditions the prohibition of his fiancée or her divorce upon her concealing something about the relationship, then she must inform him, otherwise the prohibition will occur. And the suitor should not seek out the faults of Muslims, for the children of Adam are prone to error.