There's no shame in a DNF for any reason imo. You tried and weren't equal to the task on the day. The hard part is figuring out why, and then addressing those issues so that it doesn't happen the next time.
Looks to me like your actually racing too much - assuming you are going to be racing all your races. Where is the time to recover? and the time for your body to adapt to the training stress. Personally I wouldn't schedule another race between the quadzilla and SDW50, just make sure you're fully rested and recovered and get 6 weeks decent training done, without having to lose a week to recovery from yet another race.
After SDW50 you only have 4 weeks til TP100 - at least one week recovery, and at least 1 week taper only leaves two weeks 'training'. If you get caught up in the 'must race' mentality you risk burning out.
They sound like corns. Do you have callouses(sp?) on that part of your foot? I know it's not the normal place for them, but I've had them on the outer edge of my foot, and in the middle of the ball of my foot - where they also shouldn;t be.