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I suggest using two toothpicks (or something with a similar blunt point) to pry open the shell (being careful to avoid touching the seedling itself). Try pushing outward (both at the same time) where I marked arrows on the picture below. I had to do it to one of mine and it came off pretty easily. Since you have six others and are ready to write this one off, you have nothing to lose trying to get the shell off, right?
Size-wise I had to transplant to a larger pot (from a Solo cup) less than a week after they sprouted. It seemed slow at the time but now I can't believe how big mine are after only 3 weeks!
Good luck!