Well, the final three eggs never did pip, and I candled them on Saturday night to see what was going on. By candling, you can see if there is a fully-formed chick and if it has pipped or broken through the inner membrane into the air sac, one step in the process of hatching.
It turns out that those three eggs did not contain fully-formed chicks. The last time I had candled was Day 14, so chances are that they quit very soon after that. Late quitters are often attributed to trauma during shipping. I’m not sure of what the specific sort of damage is, but it seems that the structures within the egg are damaged just as not to support the final stage of development.
So, in the end, I have the six chicks pictured. As you can see from my spreadsheet here, I had: 1 clear, 1 week-2 quitter, 1 week-3 quitter, a chick that needed help hatching, and 4 chicks that hatched on their own from the 8 Silver Ameraucana eggs. I had: 3 clears, 2 week-1 quitters, 2 week-3 quitters, and one hatched chick from the 8 Wheaten Ameraucana eggs.
Now, hatching season is over. With soaring summer temperatures and reliance on air conditioning, the temperatures in my house are not steady enough to run an incubator. Time to clean it out, pack it up, and store it away for next hatching season.
By then, I may be able to hatch my own eggs!