Can you drop the temperature more in the mudroom for a day or two? Stop heating that room. If you can get it closer to freezing for a day, then put them out in the morning. That will give them another day till it gets below zero again.
We don't have the sandy soil that you do. If our soil is dry, the chickens can dig out some decent dust bathing holes and the dry clay turns to dust but like you, it isn't always dry.
I can't really suggest a ratio of lime to sand. I think the term 'some' is what I would suggest. The lime helps...
You may be worried about the wrong things. It is extremely unlikely the two mothers will attack the other's chicks. They don't do that. Their main concern is protecting their own chicks from all comers.
The best time to introduce chicks to the flock is as soon as they can walk. The longer you...
This is old info but still worth a look. Also look up the polish bantam club for members in your area.
https://www.thefeatherbrain.com/blog/polish-bantam-chicken-hatcheries
Never add a single bird to an existing flock. It will be ostracized. As much as possible, you should add like numbers and...
Agree with @Debbie292d.
Your incubator is running at least 1 F low and likely significantly more. You are fortunate to have as many out as you do. The one pipped is likely too tired to complete the process. Whether to assist is a personal choice. If I know my temperature is correct, I rarely...
Unless it is a hen or bird that has brooded the chicks, the others will take time accepting chicks into the fllock. Once accepted as flock mates, it is rare for a bird to become ostracized, but t happens.
The pecking usually happens just to let them know who's boss. At least till they grow up...
I think it is a good choice. I wouldn't worry about off gassing. The powder coat is baked on and cured at over 300F and probably for at least a half hour, maybe several hours. The temperature it will operate at is much lower.