I totally agree with you no-stress. I think one thing that has not been brought up here is that this is NOT a legal issue. There has never been and never will be a law written in any jurisdiction which tells you what you can do with an airline ticket. It is a CIVIL question which relates to the terms of the Contract of Carriage which you agree to when you purchase a ticket from the selling airline.

Basically your question is am I violating the terms of the agreement that I agreed to with the airline when I bought the ticket from City A to City B. As long as your second trip does not violate the terms of the first ticket the airline has no recourse against you. Here again, the ONLY recourse is what the airline decides to do to you and your account IF you do violate your agreement. Basically the worst that can happen IF you violate the terms is that they charge you the difference and "fine" your FF account.

In the original scenario you are not violating the Condition of Carriage, Terms of Service or anything else, so you and the airline have upheld there ends of the agreement. Nothing to be concerned about.