Fastening stone stair treads to a CMU base in a removable manner
Stairs need to be EXTRA safe, because a loose stair is potentially deadly. It's not something you want to mess around with, heavy or not. Could you drill and bolt them toward the back of each tread, running the bolts down into the concrete below the CMU blocks? You could countersink the bolt heads  on the tread stone and fill them with sand or something to hide them, then vacuum out the sand if you ever need to remove the bolts to lift the tread. If you do that, use stainless steel hardware all around.