Severe mite infestation in winter .... AND minimal barely any mites

I posted a couple of days ago about sick hen with both sour and impacted crop and possibly egg bound. This started a couple of weeks ago. First treated for sour crop and with improvement allowed food then noticed what seemed like impacted crop. She'd go sit in a corner instead es of roosting. I isolated her in a cage inside the coop. She would get better and after two or three days I would let her out but same think kept happening. I figured she had become intolerant to pellets. Her stay in isolation, she became stumbly in weak. I began to wonder about being egg bound. I thought I felt something swollen but still not sure. I decided to try warm bath. When I went out to do bath, hen was near death. I had a hen in a similar near death state but was able to save her so I decided to go ahead and see if I could help this hen. In doing so I realized she had a severe mite infestation. She was covered. I was soon covered ... Not just my hands but boots, pants, near everywhere. After bath I knew she would like die as I realized the weakness was probably anemia and I had nothing to kill those mites. She did pass on.

This was all nighttime. I did not check the coop and other birds until this morning when I had good light.

I dealt with mites once before ... I got a used coop and had a hen very prone to going broody and she sat a lot. That only occurred one summer at least the more severe infestation. I treated often and aggressively.

The hens are in a completely new home. It is kept very clean. There absolutely no heat and no lights. They have an extra large window for natural light only. It is well ventilated, being in the northeast, we are having a pretty cold winter so fat. I am surprised to be dealing with a severe infestation .... It is far worse than the summer one I had seen.

Here's the thing ... Or one thing, I went out to examine the rest of the birds and the coops. So far they look fine, no mites. The nesting boxes look fine. The main roost looks fine. I did see what I think was 1 mite on a post.

My thinking is this hen became vulnerable because she was already a little sick and isolated. She was deprived of food off and on due to crop issues ... She was getting water and electrolytes and a little yogurt or scam pled eggs. I feel awful I did not catch this earlier. I held her many times and there was no obvious signs. But I have not held or for the past two or three days.

So, my question is how aggressively do I treat? Today I will at least clean out all shavings, I have already removed hen and cage. I will replace shavings with sand. The birds have access to sand or very fine solid under a shed they like to use, should I put a dust pan in the coop too? Should I treat the sand with DE?

Thanks

J