QEMU-KVM: how to see contents of top layer qcow2 image only (revert some back to backing image contents)?

I use kvm/qemu on Linux and usual setup is having backing image and running top image that tracks changes (qemu-img create -f qcow2 -F qcow2 -b backimage topimage). I'm concerned with growth of those top images. I'd like to be able to do something similar to how I see COW on linux: see contents of top layer as ordinary directory tree, do scan, identify large folders/locations and just delete some if I don't consider those changes important enough (which in effect revert contents to lower level image; deleting deletions restores files on resulting fs). How to do similar for top layer image file in qemu-kvm? Could not find the answer via web search. TIA I've tried rebasing top on empty (just created by qemu-img) image, but then trying mounting as in e.g https://unix.stackexchange.com/questions/268460/how-to-mount-qcow2-image (guestmount, qemu-nbd) shows only whole disk with unknown filesystem (no partitions).