By the way, straw can harbor mites!
I would get rid of it and use pine shavings.
Good luck with your girls!
I wouldn't worry too much about mites, this time of year and your location is so cold, that the straw should be fine. I use straw as it is MUCH warmer than the shavings. I layer straw and shavings on the coop floor and use the straw in the nest boxes!
If you think it might be a problem, you could sprinkle a little DE or Sevin 5% on the straw to prevent mites or lice...I don't and have always used straw with no problems.
Chickens will roost at the highest point possible...is there any way to raise the new roost above the height of the old? No, moving them at night will not hurt or upset them. It's actually the best time to move a chicken as they are mostly sleepy or sleeping and when they wake up, have no idea they have been moved LOL...that was a great suggestion!!!
As for the nest box...since the old spot was imprinted in their brains...they might not ever lay in the new boxes...making them darker might help??? Like the other OP said, once chickens imprint something, it's very hard, if not impossible, to change their habits. Best of luck and keep trying with the golf ball...ya never know...maybe at some point they will get the picture???
If you are having issues with the eggs getting dirty on the floor, you might have to make them a nest box there, if you can't get them to change. A plastic crate or 5 gallon bucket nailed to the wall should help!
Also, I wouldn't worry about the cold too much. It's been 16 below here, a few times already and consistently below zero at night for several weeks, with no end in sight...all the birds seem fine and no frostbite or signs they are uncomfortable. Chickens are more cold hardy than heat hardy. They will be fine. If you are really worried about the cold, check out the Alaska threads...they are wayyy colder than you and I are and their chickens do just fine as long as there are no drafts! A little scratch or cracked corn, fed late in the afternoon, will also help them stay warmer at night! Just make sure it's not more than 10 percent of their feed. I do feed a little more than 10% when it's going to be really really cold
Just to ease your mind, mine just started laying, they are spring pullets, in this cold, so they aren't stressed by the cold at all or they wouldn't be laying if stressed by it!
Hope this helps!!!!