1. Since we show you all the paperwork in advance, the only thing you don't know before I call is how much stock we'll ask for. Usually groups can decide about that on the fly, or even in advance. So most people either tell me yes or no when I call them, or call back a few minutes later. But we don't have any deadline for replying, so if groups have some reason they need more time to decide, we always give it to them. E.g. there was once a startup that we offered to fund that got an acquisition offer that weekend. They needed weeks to see if the acquisition was going to happen. (It did.)
2. It wasn't DST but the Start Fund, which is a separate thing. I believe those guys are going to invest again, but I can't promise anything, because it's not for me to promise. A lot of people don't realize this, but YC was not a party to that deal. It was just between Start Fund and the individual startups. (We were in favor of it, certainly, but we didn't have any agreements with anyone.)
3. In years past I caved a few times when people tried to negotiate with me, but I haven't recently. I realized I was just doing it out of fatigue at the end of a long day of interviews. I've also learned from experience that there is near zero correlation between groups' tendency to negotiate and how well they end up doing. In fact come to think of it, possibly an inverse correlation. So it's easy to say no.