They are notoriously inaccurate. And it's often the difference between life and death.
If you did have other chicks hatch on day 18... guaranteed your temp was higher than it should be. They should not hatch until day 21... with maybe a couple earlier on day 20 or as late as day 22. That is since you said the eggs came from your birds, I presume they were stored correctly and not already developed before going in.
While I agree that cross beak could be genetic.. all the deformities indicate something gone very awry.
It's only due to the hundreds of chicks I have hatched and things I have seen... never any as bad as what you describe even with power outages or old bird. Day 15 quitters would be nutritional deficit in breeding stock or at least where I would start my research. The reason I suggest culling now is there doesn't seem like a real chance at a normal life and with so many obvious deformities, I just can;t imagine the inside being fully and correctly developed... and chopping the head off with scissors (or other method you are most comfortable with) is the most humane thing to do. Otherwise it is mist like to die slowly over the next few days... starving the whole time. Learning and being able to make these type of calls is likely the MOST difficult part of hatching... but it is necessary. It isn't a recommendation that I make lightly. It comes from my personal experience with things more minor than what you describe which continue to get worse for the bird despite my best effort and care. And it gets even harder after you poured all you have into saving this cutie and STILL have to cull or watch die slowly.
The gassing method might be easier for you.
