I think feeding-until-dead can be deliberate, or from loss-of-control. It's less clear what causes the "half-turned" phenomenon... does it have to be intentional? Does it happen automatically, unless suppressed? Etc.
I think in the case of the half-turned it is a little bit of both. A good example of intentional is Susan, the impression I got from Bianca is that she did it deliberately as revenge for her secretary. Her thinking I supposed, knowing there was no cure that a half turned Susan would make Harry suffer for a very long time. However Justine was spared, was this simply because they hadn't had a chance to either feed upon her or half-turn her? Or since she was the girlfriend of Thomas they didn't want to risk pissing off the White Court? In other cases motives for half-turning a victim is less clear, especially when many go on to join the Fellowship of St. Giles which is an anti-RVC resistance group devoted to limiting the damage that the Red Court does. So why would they do that?
Harry managed to coach Susan enough that she could hold it off, but she wasn't able to consume any blood to sate the hunger, so the urges were getting worse and worse. We don't know how Susan managed to get her hunger under control after the climactic battle in Grave Peril, before she left Chicago. It seems very likely to me that the Fellowship contacted her there in Chicago, in the aftermath of the Velvet Room burning down (they likely had Bianca's under surveillance, because of all the Red Court activity surrounding her elevation to the nobility; they might have picked up Susan's trail there). This is yet another open question in the series.
I don't think Harry did anything to coach Susan in resistance to the venom. I think controlling hunger, emotions, impulses connected with newly born "super powers," i.e. strength, speed,and longevity take time to kick in. I agree that the Fellowship more than likely had Bianca under surveillance and perhaps even Harry as well, on the look out for the half-turned. I believe Harry was in the hospital for a bit, and during that time the Fellowship might have made contact with her, Yes, Susan was of Hispanic heritage, but I don't think it mere coincidence that she fled to Central American where the Red Court strongholds were and where the Fellowship is the most active.
Changes
Thanks, I reread the passage and it is very interesting from the standpoint of while mentally/emotionally he got excited about the prospect of Lea having a way to cure both Susan and Martin and possibly all the half-turned, he didn't leap to the prospect of agreeing to a bargain with her. Not I think because he didn't want to cure them and no doubt would give up a lot for a cure, but because he knew from her answer that she wasn't really offering a cure.. She said vaguely that such knowledge would really be a "treasure." She was willing to bargain "knowledge," but she wasn't specific as to what that knowledge was... The Fae cannot lie, but the reason why it is so dangerous to bargain with them is that they are willing to let you lie to yourself! She agreed that such knowledge was very valuable, but she never said what the knowledge was she had, or that she had any knowledge beyond putting them asleep. Harry had already spent a year searching for a cure, Bob had told him there was none. While putting a RCV or half-turned RCV to sleep was useful, it wasn't all that useful and Harry realized that. On the subject of bargains it is also interesting as to how much Lea really did do to keep Harry safe as per her agreement with his mother.. Also the comment about Margaret not being able to sleep after she acquired the roadmap gem of the Nevernever and how dangerous it was.. Why would that be?