Is it permissible to speak with a former suitor regarding the renewal of the engagement, knowing that he has become engaged to another woman, and would this be considered an injustice to her?

It is permissible to break off an engagement when there is a need, for example, if the girl dislikes the suitor. There is no sin or injustice in this, because it is a contract where harm would persist.

The general rule is that it is not permissible for a young woman to speak with a non-mahram man without necessity, as it is a cause for fitna (temptation/strife).

Therefore, it is not permissible for you to speak with the suitor in a way that might turn him away from his fiancée, as that could be considered 'takhbeeb' (corrupting a woman against her husband or fiancé).

Turn away from this and ask Allah to grant you a righteous husband.