For the toenail thing, one possibility is athletic tape. I have used this in the past if I had any issues.
Pace could definitely do it. Even if your form is identical at different paces (highly unlikely) you are applying higher amounts of force.
For the short term, I have to say that I agree with Steve. We’ve all had blisters, I’m dealing with a nasty one right now that tore open yesterday but I just get out there and put it out of my mind. I usually don’t even notice it after the first few steps. Still, I understand the desire to try to figure out why it happened so you can prevent it in the future. While blisters are almost always the “suck it up and get out there” type of pain, it sure would be nice to minimize the frequency of getting them.