Most of my Aquascape is glued and epoxy. I drilled holes where I knew I would place frags.
I have shelves & rocks in place that are not glued. Them, I can pull out at a water change. 2, I cut the frag nipples, then glued them outside the tank.
The tank back, I made magnet rocks. I pulled 1out to drill for a Green Star Poylp. I have 4 more coming. I'll do the same so I can cover the back glass.
2 flat rocks, in the front, just lay on the substrate for possible invasive corals.
Looking at your rock work, you can cut the nipples off, then glue the flat frag to the rock at a water change or remove & drill. Either/Or
They cut easy with wire cutters.
I read some time ago that if you want to place a flag below the water line, you put a dab of epoxy on the flat frag, then glue on top of the epoxy. It said the glue will form a bubble cover over itself. When you press, the glue bubble breaks, adheres the the epoxy and should hold your flat frag. I've had no need to try it that way.
I hope these ideas help you