Trouble understanding how Profiles and Smart Groups are supposed to work together
Hard to know quite how to unpick what you have posted but the bit that stopped my eyeballs in mid-skim was this


  setting the "add to smart group" field of, say, the Volunteer profile to "Volunteers"


Not sure if you have the concept but a Smart Group IS the results of the Search used to find the people who should be in it, ie it is a set of criteria, not a set of contacts.

By adding contacts to a Smart Group via a Profile means they ARE in the Group, but they are not SMART - ie you are not using the 'criteria' to determine if they are in the Group or not, you are force adding them (which is allowed for Smart Groups but you have to understand the distinction)

Not sure if that helps