A few details about smart lists and how they work.
How does the sync process work?
When a list is synced to a report, the results in the report will replace what ever is in the current list - it does not update, but replaces the contacts on the list.
Will we be able to tell when a contact was originally added to the list?
The join date to that list/committee will show as the date that the report ran - it does not keep an original join date. That being said, you may be able to find the original date be reviewing the history on the Activity tab of the individual's contact record.
Can we have a synced list that includes both organizations and individuals?
If an org is on the list, that doesn’t work well - report results should be individual contacts. Please structure your reports accordingly.
What time do the updates take effect?
The automatic Smart list updates seem to occur sometime in the evening hours i.e. 7-9 pm CT.
How can we tell if a contact was added to a list by a staff person vs when a contact is added by the sync process?
The addition/removal of the contacts that is done by the Smart Lists is shown in the Activity log as being done by the Organization Name.
If a contact unsubscribes from a synced list: when the list is updated by the report, does it "remember" the the contact's selection?
In a word, yes. If a contact unsubscribes from a list that is being updated by a report, the list will remember that the contact selected to unsubscribe from that list.