It's a learning curve and crap happens. Next time remember the heat issue and have a tested solution ready to go for next run.
I would try dampening the shell first. Strong, vigorous and healthy plants are what you want. A plant that fits that bill will shed it's helmet in the right conditions.
If that does not work in 24 hours or so you can pull the shell off. Someone on here may have a good way of accomplishing this, I do not. I do the best I can to hold the seedling gently in place and split the shell as carefully as I can. Take note that once the shell is off a layer of clear film can stay over the cotyledons, if they do not open in 12 hours or so, check that you were able to remove that.