Re: Taliban cut off heads of 17 people for having music at party 
To apply a sentence of death on anyone, requires a degree of evidence which seems almost unobtainable. Short of filming the act itself, I dont see any way one could prove, beyond a shadow of a doubt that two people are engaging inromantic activity. That is assuming adultery is not broadly defined as ANY contact between two unmarried people. Any evidence then that does not prove that they are romantically involved then would simply be conjecture, and basing a sentence of death on such evidence would just be pretense and so unjustifiable. So yes, it is a very difficult thing to prove.
But as you yourself said, the sentence of death by stoning is extreme. And I think the belief that it works as a deterrent is more an assumption then something based on actual fact. Human nature i understand, and one thing about amorous couples, they never allow the fear of death stand in the way of their “love.”
And on a separate note, while the vows of marriage are sacred, are those vows still considered valid if your marriage is one that is forced? So you know as well as i that in many Islamic (and non Islamic) countries, you do not have the choice of partners, despite the assertion by your faith that you do. Now the pressure to marry a particular person can be great. The nikkah ceremony may ask give you that final option to decline the proposal, but how often does that happen considering your entire “khandan” is behind you? So does application of anti Adultery laws in Islam, not assume that the marriage is by mutual consent without pressure and the option of divorce is available, even though it often is not in the case f many traditional marriages?