Is breaking off an engagement due to the discovery of a moderate heart valve problem in the fiancée considered an injustice to her, for which the suitor will be held accountable as a breach of promise?

It is permissible to break off an engagement if there is a valid reason. If there is no reason, it is permissible, though disliked. You are not at fault for leaving this girl, and it is not considered an act of injustice or abuse. However, if you complete the marriage and patiently endure her illness, it is better and will earn you great reward if you intend thereby to please Allah.