Smart Groups - Contacts by Adder
This is what I have discovered through further trial and error.


To search on Changelog by Added or Modified, use first name, last name, or last, first. Using both names separated by a space will not work. 
The Changelog search actually makes no distinction between Added and Modified. Even if you select the Added radio button to restrict your search to Contacts Added by a particular user, it will still also return Changelog matches on Modified (which does include Added). In other words, whether you select neither radio button, or one or the other, you get the same search results, in which Added OR Modified contains the name you entered.
The case of the search name makes no difference in results returned.


The answers to my question, then, are that a Changelog search term may be in the formats of [first name], [last name], or [last, first] and that Changelogs are only filtered by Added OR Modified, despite the UI indication.

Also note the very helpful discussions here about using Webforms with Drupal do create many more options for filtering by contact Add and Modify history.

Kathy