I would try and get the treads up and repair them from underneath. I would then fit a bracer in the void to give me something to screw the repaired tread into. Also would use screw-tites for no squeak.
I have a small pantry kind of thing under the stairs , storage basically, looked today and I can see a few of the stairs through the falling down plastered ceiling..its got those old wooden strip type of boards..I can easily pull those out and basically put some thin mdf as a ceiling not really bothered about how it looks , it's under the stairs.. ..what are screw tites. ? What screws do i use to prevent treads riding up and squeaking? I remember using some black screws years ago , got them from screwfix to screw in some squeaking floorboards..ta