Many seedling problems come down to watering. It commonly seems to be over watering whenever someone just whatever's every "x" days. What works best for me with seedlings is a small medicine dropper that I can get 3, 4, 5 etc. ml of h2o out of at a time and when I see the soil is dry I water...
Fishing, gone for a couple weeks. Guessing orders would back up, enjoying some time off is nice and coming back to a pile of work is stressful and poor customer relations to those few jerks who just don't understand.
With your RH that low they should be drying out pretty quickly... Have you been letting them dry until the pots are light or watering every day regardless?
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...
It is possible to carefully remove the shell, just have to be cautious to not damage or remove the seedling from your medium. Do you have them in a dome or some place that is able to keep the humidity elevated?
That would be where I start as well. A good flush. Let the soil dry and then give it a good watering, watching the plants response. Post some photos back here if you see things you want more help with.
Not sure where you are and what temps are like, but, are you bringing them inside in the evening/think you have a need to? Were they started inside and hardened off before being moved outside?