On the profile, there's actually a Permissions*PermissionName* field for each permission that you can query against. It's using the field's API name.
In this case, you're interested in Modify all Data so the following SOQL gives you that
SELECT ID,Name FROM Profile WHERE PermissionsModifyAllData = TRUE
In terms of permission names, you can refer to this ...
Based on this:
taken from the Profile and Permission Objects documentation and noticing that a PermissionSet can optionally be tied directly to a profile - presumably the permissions are defined for a profile via a hidden PermissionSet - I would guess that "yes" you can query the complete access permissions.
On the subject of what is the most a ...
I assume you are using Lightning experience to view the "Page Layout Assignment".
I could reproduce the behavior you mentioned i.e., Page Layout Assignment doesn't show the Guest user's profile on my org too in LEX. However, when I switched to the classic view, I could find the Guest user's profile which in my case is named "Napili Site ...