There are so many things I don’t know about your situation it’s hard to speculate. You may already know all this but you said it was your first time with a hen.
A broody hen needs to be able to cover all the eggs. If the bantam was covering eggs the size she lays, she was probably OK with that. If she was covering eggs laid by full-sized hens, that may have been too many. If the hen cannot cover them all, you usually get a pretty bad hatch rate.
You need to start all the eggs at the same time. It’s possible others laid with her or something like that so some haven’t been under her the same length of time. I mark all the eggs I want a broody to hatch and put them under her at the same time, then check daily and remove any that don’t belong.
The night-time lows aren’t all that important. The hen’s heater is set at 100 degrees. She can keep the chick and eggs warm. There are some photos on this forum of a broody hen with her chicks out in snow. It is harder for a broody to raise her chicks in the winter when it is cold. Bad things are more likely to happen. The colder it is the more likely it is something bad will happen. But those chicks are tougher than many people think. They’ll often surprise people.
I’m a little surprised the hen has not abandoned the eggs and taken her one hatched chick to find food and water. The chicks absorb the yolk and can live off that for three days or so but soon it will need food and water. On rare occasions you will get a broody that will stick by the nest if she hears chicks peeping in the eggs, but practically all of them will choose to take care of the already hatched chicks. They are living animals though so I can’t give you any guarantees.
I think you have done about all you can do. If you mess with the broody she is really likely to abandon that nest. If you get a chance you can try the float test. Put the eggs in a bowl of still water. At that age, if something is alive in that egg, the egg will wiggle. If it is very still, nothing is alive.
If some wiggle and the hen abandons the nest, you can try putting the eggs in an incubator of you have one, or a brooder or a box set at 100 degrees to see what happens. It’s a rough situation to be in. I wish you luck!
