I set a top group as private with sub groups/galleries as public so that I can send a link without having the pics viewable from my home page. However, all the public group/galleries are automatically set to "Site-wide searchable" within Access Control. This means that all the photos that I only want a select few people to be able to see are really viewable by anyone on zenfolio.... or really.... google.

I'm now going through group by group, gallery by gallery, in order to change these settings. So, my second question is: Is there an easy way to select a large set of groups/galleries and set their Access Control settings all at once? It's taking a long while to go through and do it one at a time. 

 sure!  in control panel, go into your private group, and then select all the galleries you want to change...  you know how to make multiple selections?  hold Ctrl while clicking (this is windows, not sure about mac) and it will select multiple galleries.  then go into your access control and change the setting for search to what you want and it will change them all to that. easy!