Is it obligatory to fulfill the marriage vow to a girl with whom a relationship lasted six years, with sins committed short of major ones? And is the motive of expiating sins sufficient for proposing to her, especially with the warning that such relationships can be a cause for the destruction of marriage, knowing that she does not represent the ideal wife in terms of religious commitment in the questioner's view?

It is not a condition for the validity of repentance to marry the person with whom Allah was disobeyed; rather, repentance is sufficient to expiate sins. If the girl is not righteous, it is not appropriate to marry her, and one must be keen on marrying a religious woman. However, if she has repented to Allah, she is worthy of fulfilling the promise, and that relationship has no effect on the success of the marriage, Allah willing, because repentance nullifies what came before it.