Eggs too hard to hatch?

The shells of eggs laid by our chickens have always been harder than the shells of eggs you get in a supermarket, and I understand that's totally normal.
We have been incubating fertilized eggs, and have hatched two so far (it was a highly staggered hatch). Unfortunately, we had a big temperature spike (+104 degrees overnight) in the bator. The eggs are a very dark brown and are somewhat difficult to candle. About 3 days after the spike, I did a water float test to check for viability. Sadly, about half of our eggs failed the float test.
I wanted to find out about how far along they were at the point where they died, but had a really, really hard time breaking open the eggs. Like, really hard. Like, wow, this is ridiculous, I can't hardly break these eggs.
Also, with the first two hatches, one had to be assisted because she couldn't seem to get through the outer membrane, which was really tough. I ended up pulling the outer membrane and picking the shell just enough to give her room to get out.
I have wet sponges in the bator and maintain a relatively high humidity. Is it possible my eggs are too hard to hatch? Is there something I can do to help with that? Having just lost so many, and realizing how hard the eggs are in the process, I would hate to have any more issues. Thanks for any insight!