I see several issues brought up here.
One is the nature of forgiveness. While I am indeed a Christian (of the Eastern Orthodox kind, more on that later), methinks the relative value of it is not particularly germane to the story at this point. Nicholas deliberately gave Angelique an impossible task. At this point, it would be utterly impossible for Barnabas to forgive her. Consider for a few moments the enormity of damage she had done in his life, not only to himself but others. Not even once in 1795 but again in the (then) modern day. Quite apart from the fact she isn't really showing any repentance, only a personal terror at danger to herself, Barnabas himself is too filled with rage and self-loathing to do anything but aim it in her direction. At this point in their lives, he is actively trying to stop Angelique/Cassandra from inflicting all kinds of suffering not only upon Barnabas himself, but upon those whom Barnabas himself victimized--a fact for which he himself feels both intense guilt and seething fury because he (with some justification) blames her for his actions. After all, she was the one who made him a vampire! So the guilt fuels his anger.
Years later, after Barnabas had more time with which to ponder events, seen a much more vicious version of Angelique (in parallel time), and frankly come to grips with his own culpability in things like [spoiler]Rachel Drummond's murder and the death of PT Willie Loomis,[/spoiler] then he was able to forgive. And at that time Angelique did far more than ask for such. She tried to atone by saving Barnabas and others, by even putting her own neck into a noose without any need save the desire to save the innocent. Recall that she could have fled at that point. More, at that point recall that Barnabas understood more about Angelique, about how someone specific had corrupted her--and that someone was out to destroy Barnabas' friends.
I will mention parenthetically that in my Church, forgiveness per se is not so much a path to salvation but a symptom of it. We believe all enter God's love upon our deaths, but only those whose hearts are open experience it as love. Those who do not, immersed as they are in what they reject, experience it as a hell.
But now turning to Diablos and Angelique/Miranda. Somehow I don't think Miranda was all that far-seeing or, more importantly, far-thinking when she made her pact. Nor do I think she had any direct contact with Diablos at that time, but rather seemed to be a disciple of Judah Zachary. For all her cleverness, Miranda/Angelique seemed to me a creature of the moment rather than one with particularly long-range plans.
She and Barnabas were alike in this. And they were alike in something else. The unstated but important fact in this episode is that Angelique at this point still has not forgiven Barnabas for rejecting her. More, he became her lover without intending to keep her, to continue loving her. Frankly, she had a point. His sins against her were of a different magnitude than hers, but that does not negate them nor the pain she suffered as a result. And from a purely chronological point of view, isn't Barnabas to blame for what happened? Because he used a serving girl in a callous, thoughtless way but then had to live up to consequences beyond his wildest dreams? (Fatal Attraction anyone?)
My point is that forgiveness between those two needed to flow both ways, and until each of them realized that on some level it couldn't happen. Might as well demand someone breathe molten lava or go to moon by flapping their arms.
JMHO
