So, one easy way you could implement this as the default for your public users would be to use the Menu module to change the default path in the Browse > archival descriptions menu, from
informationobject/browse to
informationobject/browse?topLod=0 :
I just tested this out on our public 2.2.1
Demo site, and it works as expected - when users click Browse > Archival descriptions, they are shown all descriptions. They can always use the top-level filter button in the facets to restrict the browsing to just top-level descriptions again.
Hope this gives you what you need! Otherwise, you'd probably end up having to change things in AtoM's code.