Have you considered 3M VHB tape? Expensive, but it is holding many solar panels on the roof, mine included. Various widths are available. I have also used it on lighter-weight applications, and it works very well. Nothing has fallen off.
On the flip side, if you use Z-brackets to mount your shelves, you could put two screws in the wall for each bracket. More support that way. Or, get the hook-and-plate mounts (I have a pair holding up a cabinet on the inside of my trailer). Each mount has 4 screws. That gives a mostly-down force on the mounting brackets, so they are stable.
Just my thoughts.