I don't think that a moderator has access to "become" a member with the "edit user" permission in reguards to access to the CP; I think you'd have to grant them the "Full Access" permission to do so (which I dont' know anyone would do such a thing for a "mod").
v7.3+ (I think it was 7.3 or 7.4) there is a built in "Global Moderator" position which is like an admin without the CP access; there are also granular permissions for just about any task so you can ultimately customize groups with what type of access you'd want people to have who belong to them.
In short, just setup the global moderator group to have edit user permissions and assign people to that position to weed out trolls; as long as they don't have "full cp access" permissions enabled they won't be able to "become user" and read PM's.
