] I think Darcy's comments could be read either way. Darcy doesn't explicitly say who initiated the conversation,

No, he doesn't but the way he expresses it does suggest that he did. Certainly he returned to London intending to.

] and I think it's important to remember that Darcy is upset while he's writing,

How upset is he? In P&P2, we see him returning to Rosings Park immediately to write the letter. I can't find the passage now, but I seem to remember that there was something in Chapter 34 or 35 that indicated that he did not write the letter until morning. In that case, he had hours (I doubt he slept much) to think about how he could most effectively lay the case before her.

] he thinks quite highly of what he has done, and he's trying to persuade Elizabeth that he isn't as terrible as she believes (i.e. if she could see things from his perspective, she might not judge so harshly and could agree with his point of view). So, I would expect Darcy to be persuasive in his writing and to perhaps overstate his actions and influence, since he believes himself to be justified.

But he is, as you say, trying to persuade Lizzy that he isn't as terrible as she believes, and so I still think he would be likely to include anything that seems to mitigate his behavior -- and being asked for advice would mitigate it. Also, overstating the strength of his represetations to Bingley does not help his case; in fact, it makes it sound worse.

] In his behavior toward Bingley, however, I don't think Darcy would be quite so bold. Yes, he can influence Bingley, but Bingley isn't a child

However, Bingley is, we were told, ductile.

] and I think if Darcy were too strong in his criticism of Jane, that would just anger Bingley and decrease Darcy's influence. A quieter, slyer comment that has more of a questioning tone ("surely you can't be serious about her?") would sow seeds of doubt in Bingley without putting him on the defensive.

"Surely you can't be serious about her" would do about as much to put someone's back up as any strategy he could have chose, I would think. But again, I don't see how overstating the way he expressed his opinion helps his case, and it would likely have hurt it -- something he could surely see.

] I know P&P2 has the scene where Darcy and Bingley's sisters have cornered Bingley and are pretty much scolding him into forgetting Jane, but that doesn't ring true to me.

Me, either, but as B. Michelle pointed out, I think that was intended to represent the way Lizzy imagined it.

] Bingley may look to Darcy for guidance, but he's a grown man and I don't think he would put up with being scolded like a child.