If you think about it, there isn't going to be any trustworthy sources looking back on it, it's simply just an obscure law. If you were Lord, you wouldn't want all of history knowing your horrible secret, therefore you would most likely hide it from the history books. Only unofficial scholars would actually document anything about it, which leads to people citing it as a myth.
First of all, such a law would really tick off the Church. They were tracking details such as who was Bishop of Greenland at the time. A law passed in England by a nominally.Christian King that gave the finger to the sanctimony of marriage would be much better documented by the Church than the 18th Bishop of Greenland.
Secondly, the stuff that happened in this period isn't as obscure as you think it is. We know what Edward fought with his parents about when he was doing the teenage rebellion thing. We know the dates on which he arranged tournaments. Etc. etc. etc. We know what laws he passed, and we would especially know about such a scandalous law as this.
Thirdly, by its nature, a law can't be a secret.
Fourthly, rulers generally don't consider the things they do shameful (no matter how shameful those things may seem to outsiders).
Fifthly, what's in this for Edward? He and his wife had 15 or 16 children. They
liked each other. If he had wanted something outside of marriage, that sort of thing's not hard to get for the richest and most powerful man in the kingdom. And he spent very little of his time in Scotland. So your alleged law wouldn't benefit him. It would "benefit" the men who were sent out to administer (read: collect taxes from) the Scots, who would be roughly the equivalent of middle management types today. So why would a king, who was generally one of the most competent of England's kings, pass a really, really, stupid law such as this, just so middle management types could rape Scottish women?
Oh, and sixthly, Edward conquered Wales before he invaded Scotland. So why wouldn't he pass this law in Wales, too?
We know that Edward was constantly getting rebelled against in England because of all the taxes he tried to enforce in order to pay for his wars in Wales, Scotland, and France. We know that he tried to
draft the Scots to help fight his French wars. These are more than adequate motivations for the Scots rebellions. The idea that the English decided that raping the wives of the men whom they wanted to protect their backs in their French Wars was a good idea is ludicrous.