Please let me know if I'm right in presuming that a response to a letter with "per pro" (typed by the secretary for the boss) should be addressed to the boss and not the secretary. And the boss's name, not the secretary's should be used in the inside address as well. I need a double-check on that.
Correct on both counts. The "per pro" person is akin to a ghostwriter; the thoughts/message they are conveying are not their own, but those of another party. So any responses should be addressed solely to the signatory of the letter.