“In an effort to be halachically correct, how many lives have been altered in a terrible way?”
No one’s life can ever be “altered in a terrible way” through keeping the halacha. There are situations in which a person is allowed to say something and situations in which one is not.
Everything comes from Hashem. He is the one who makes everything happen. We just have to follow His rules. He made the rules and He determines the results of your actions. He will never make you lose out for following His rules, although it may seem like it sometimes.
Even if not speaking Loshon Hora will seem to lead to someone’s having a bad marriage, one must believe that he will not lose out ultimately for having kept the Halacha.
That being said, generally speaking, keeping the Halachos of Loshon Hora also includes speaking when you are required to as well as refraining from speaking when you are not allowed to. So generally speaking, if you really do know for a fact that it would be harmful for person A. to marry person B, you are probably required to do so.
Of course each situation is different, and one must ask a sheilah. But the fact is that it is generally almost impossible to know how things will turn out. There have been many marriages that seemed perfect and ended in disaster and many marriages that one would have thought would be disastrous and they turned out to be nearly-perfect. There are plenty of people with serious issues who have wonderful marriages. There is usually no way for anyone to know how things will turn out.
On the other hand, if you know that one of the persons really has a very serious problem that would clearly make him unfit to marry the other person, you probably do have an obligation to say something and to try to prevent the person from marrying him (but you have to make sure your facts are correct, and you should probably ask a sheilah before saying anything).