I suggest that it is a mistake to connect Susie Cleland, who was burned, or Bonnie Annie, whose back was broken, with "adultery." This is so for two reasons.

1. There is nothing in Susie Cleland about sex.

2. In Bonnie Annie, neither party was married, so, no adultery.

3. There are lots of references, from several cultures, to the practice of killing women for failing to marry as their famly wishes, conduct which has much more serious consequences than mere adultery. I believe it was Dante (possibly Macciavelli) and I regret not having a citation, who deplored the widespread practice of poisoning "our sisters and daughters" who refused to marry as directed.

4. It is prettiy well established that in contemporary Rajastan to burn women who fail to bring an adequate dowry. I wouldn't be surprised if something like that applied in mediaeval Scotland, especially to the upper classes.

5. If someone actually knows the answer, it would be interesting to hear it.