Would I be sinful if I strove for the marriage of two people and violations of the engagement that I had expected resulted from that?

Striving to marry Muslims is a righteous act of cooperation in piety and righteousness, and it is something encouraged by the Shariah; as Al-Ayni said: "Because it contains the meaning of worship; for marriage is the sunnah of the prophets and messengers, and in it lies the attainment of half of one's religion. Reports and narrations have been consecutively transmitted warning against those who shun it and encouraging those who desire it." The questioner's endeavor in this marriage is a good deed for which she will be rewarded, and she will not be sinful for any transgressions that may occur during the engagement period; because her endeavor is not to assist in something forbidden, and the transgressions are not an essential part of marriage. That which is not essential should not be prevented as a precaution against evil means, as Ibn al-Qayyim said: "The means to forbidden acts and sins, in their dislikedness and prohibition, depend on their leading to their ends and their connection to them." However, if the questioner becomes aware of a reprehensible act, she must forbid it according to the Shariah guidelines.