I have remounted many mites in Hoyers over the years, including a few types. Place the slide part with the specimen in a container on a screen over water (not in the water!). After a day or three, the medium will soften to the point where the cover slip can be removed and the specimen remounted on another slide. Hopefully, the specimens on your broken slides are not too small, then it becomes iffy. I don't know if Cobb slides are still made - those were metal holders that held a double cover slip (square) over a hole in the center. After the Munich museum was bombed in WWII, the mite collection of H. Vitzthum was salvaged and remounted in these containers. Otherwise Jim's & Mike's solutions should work.
That's the nice thing about Hoyers. Years ago I received some mites from a colleague in Australia (where they used PVA only). Slides arrived smashed, and nothing I tried would dissolve the PVA.
Stay well! - Barry