I have figured how to go into my Messages folder, select a message, pull the menu to Search Sender, and end up with a results list of nothing but messages from that sender. Now, should I choose to Delete Prior from that list, will it delete only messages from the selected sender prior to the selected date, or will it delete all messages prior to the selected date?

Your first guess is right. I do it all the time.
It will delete only messages from the selected sender prior to the selected date as long as you are in the search screen.

That works! Thanks.