Here is the solution that finally works for me, with the help of @joequerty
pointing me in the right direction:
assume a folder sh to be shared
1) share folder sh "read only" for Everyone (\\host\sh)
2) give read/execute permission on sh for Everyone, but apply to files only.
assume we have a file in sh\a\b\c\f.txt
3) give only to (immediate parent) folder c
Read/Execute, Read and List Folder permissions for Everyone and apply to This folder only
Now \\host\sh\a\b\c\f.txt is accessible to Everyone
dir \\host\sh\a\b\c will list f.txt
dir \\host\sh\a\b and higher up will lead nowhere